3d heteroatom‐doped carbon nanomaterials as ...via chemical routes.[2,16,23,36,40] go is generally...

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3D Heteroatom-Doped Carbon Nanomaterials as Multifunctional Metal-Free Catalysts for Integrated Energy Devices

Rajib Paul, Feng Du, Liming Dai,* Yong Ding, Zhong Lin Wang,* Fei Wei, and Ajit Roy*

Dr. R. Paul, Dr. F. Du, Prof. L. DaiDepartment of Macromolecular Science and EngineeringCase Western Reserve UniversityCleveland, OH 44106, USAE-mail: [email protected]. Y. Ding, Prof. Z. L. WangSchool of Materials Science and EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332-0245, USAE-mail: [email protected]. F. WeiDepartment of Chemical EngineeringTsinghua UniversityBeijing 100084, ChinaDr. A. RoyMaterials and Manufacturing DirectorateAir Force Research LaboratoryWright-Patterson AFBDayton, OH 45433, USAE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201805598.

DOI: 10.1002/adma.201805598

states, such as sp, sp2, sp3 or with other nonmetallic elements, generally called het-eroatoms. This feature provides a variety of carbon allotropic structures, from single molecules to layered structures and to complex mesoporous architectures.[1–3] Carbon was first identified about 200 years ago within organic molecules and as building blocks for biomolecules. Later, natural carbon was discovered in the form of diamond, graphite, and amorphous carbon. Diamond is a very well-known hardest and transparent electrical insulator but a thermal conductor. It is a form of tet-rahedral sp3 carbon atoms. Diamond has a unique crystal structure consisting of two interpenetrating face centered cubic Bra-vais lattices. On the contrary, graphite is soft and opaque having attractive electrical conductivity. It is basically a stack-up form of atomically thin (0.335 nm) graphene layers that are weakly bonded by van der

Waals forces. The graphene monolayers are hexagonal packed lattice of sp2-hybridized carbon atoms.[4,5] Clearly, the diversity in atomic arrangements can provide various allotropic struc-tures of carbon, which are different in physical and functional properties.[6–11] Motivated by this fact, there have been seminal quests for inventing new nanostructured carbon allotropes with various dimensions, such as 0D, 1D, 2D, and 3D.

The recent discoveries of carbon nanomaterials added new members to the carbon family. In 1985, fullerene (C60) was the first discovered as carbon nanostructure with 0 dimension.[12,13] About 6 years later, another 1D carbon allotrope was discov-ered, called carbon nanotubes (CNTs), whose structure was proposed by Iijima.[14,15] In general, CNTs (both single- and multiwalled CNTs, abbreviated as SWNTs and MWNTs) are utilized as reinforcements in composite materials having dif-ferent polymer matrices and as building blocks for numerous catalysts and electrodes.[16,17] In 2004, the most widely investi-gated and used 2D carbon nanostructure, called graphene, was experimentally evidenced and characterized by Geim and Novo-selov, although it was predicted and identified decades ago.[1] The graphene family type is ever increasing since its discovery. Different manufacturing processes yield graphene products diverse in size, thickness, and impurities. Thereafter, research interests on graphene quantum dots (GQDs) or carbon dots (C-dots) have been reported till now. Structurally, GQD or C-dot is graphene of mono- or few-layers with distinct but interesting

Sustainable and cost-effective energy generation has become crucial for fulfilling present energy requirements. For this purpose, the development of cheap, scalable, efficient, and reliable catalysts is essential. Carbon-based heteroatom-doped, 3D, and mesoporous electrodes are very promising as catalysts for electrochemical energy conversion and storage. Various carbon allotropes doped with a variety of heteroatoms can be utilized for cost-effec-tive mass production of electrode materials. 3D porous carbon electrodes provide multiple advantages, such as large surface area, maximized exposure to active sites, 3D conductive pathways for efficient electron transport, and porous channels to facilitate electrolyte diffusion. However, it is challenging to synthesize and functionalize isotropic 3D carbon structures. Here, various synthesis processes of 3D porous carbon materials are summarized to understand how their physical and chemical properties together with hetero-atom doping dictate the electrochemical catalytic performance. Prospects of attractive 3D carbon structural materials for energy conversion and efficient integrated energy systems are also discussed.

Integrated Energy Devices

1. Introduction

Carbon atom is unique in forming covalent bonds, both σ and π, with neighboring carbon atoms in various hybridization

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optoelectronic properties.[18,19] Although these low-dimensional carbon nanostructures (0D, 1D, and 2D) have been adapted for many functional applications due to their excellent intrinsic physicochemical properties, they are often suffered from infe-rior extrinsic characteristics, such as poor electrical and thermal conductivities, low effective surface areas, and unreliable mechanical properties.[20–25]

To address this problem, hierarchical carbon materials having porous structure in three dimensions are being pursued. Such 3D mesoporous carbon material construct possibly would pro-vide a desirable backbone support of enhanced surface area for catalytically active sites, a multidimensional conductive network for improved electron transport, and an expanded volume to accommodate the electrolyte/reactant diffusion with significantly improved mechanical stability. To date, most of the reported 3D carbon materials are basically agglomerated or simply assembled from lower dimensional carbon nano-structures, which are widely used for electrocatalytic applica-tions,[16,26–28] though there have been a few successful reports on fabricating isotropic 3D porous electrodes.[29] Heteroatom doping in carbon nanostructures is also crucial for increasing the efficiency to realize multifunctional electrocatalysts.[30] In fact, multifunctional 3D nanomaterials are promising to fabri-cate various integrated devices for fulfilling the soaring energy requirements, as well as infuse technological advancement toward efficient, reliable, multifunctional, portable, and flex-ible devices.[16,31] Here, we provide a comprehensive and critical overview on mesoporous 3D heteroatom-doped carbon nano-materials as multifunctional metal-free catalysts for integrated energy devices by summarizing and discussing advancement in fabrication methods of undoped and heteroatom-doped (multiatom doped) 3D carbon catalysts from lower dimen-sional carbon nanostructures. Also, we discuss isotropic 3D carbon structure fabrication for efficient multifunctional energy conversion and storage applications, along with illustration of various factors influencing their performance and applicability.

2. Different Allotropes of Carbon

Allotropes are different structural forms of an element. Var-ious allotropic forms of carbon with different dimensions, such as 0D (e.g., fullerene, onion-like carbon, C-dot, GQD, nanodiamond), 1D (e.g., SWNT and MWNT, nanohorns, par-tially and fully unzipped nanotubes), 2D (e.g., monolayer and multilayer graphene, amorphous, graphitic, and diamond-like carbon films), and 3D (e.g., graphite, diamond, and pillared graphene) carbons, are depicted in Figure 1a.[1,32] At present carbon-based 3D mesoporous metal-free catalysts have become very interesting for different electrochemical-reaction-based energy storage applications.[33–35] Such mesoporous 3D carbon can also be fabricated using any of the above-mentioned indi-vidual or combined allotropes. However, pillared graphene is an ideal and isotropic 3D mesoporous structure predicted to possess superior charge transport properties to other 3D carbon structures obtained through simple physical or chemical combination or agglomeration.

Graphene oxide (GO) is the latest, but the most popular gra-phene flake, which is cheap, soluble in different solvents, and

primary choice for fabricating 3D hybrid carbon electrodes via chemical routes.[2,16,23,36,40] GO is generally prepared from graphite flakes through intense oxidation using KMnO4 and

Rajib Paul studied with a bachelor and a master degree in physics, and a Ph.D. degree in materials science and nanotechnology from Jadavpur University, Kolkata, and joined as a postdoctoral researcher at Birck Nanotechnology Center in Purdue University before moving to Case Western Reserve University. He has extensive expertise in synthesis and nanostructuring of mesoporous carbon materials using various plasma induced

and thermal CVD for efficient energy storage and conversion, mechanically stable hard coatings, and optoelectronics applica-tions. His research interest is also extended to characterizing ther-momechanical properties in 3D hierarchical carbon materials and structural battery fabrication for unmanned electric air vehicles.

Liming Dai is a Kent Hale Smith Professor in the Department of Macromolecular Science and Engineering at Case Western Reserve University and is also the director of the Center of Advanced Science and Engineering for Carbon (Case4Carbon). He received a B.Sc. degree from Zhejiang University in 1983, and a Ph.D. degree from the Australian National University in 1991. Before joining the CWRU, he was with CSIR, Australia,

the University of Akron and the University of Dayton as the WBI Endowed Chair Professor. His expertise lies across several fields, including the synthesis and functionalization of polymers and carbon nanomaterials for energy and biorelated devices.

Ajit Roy is Principal Materials Research Engineer at the Materials and Manufacturing Directorate, Air Force Research Laboratory (AFRL). His research expertise includes functional materials, materials modeling and processing, 3D nanostructure, 3D composites, carbon foam, and carbon–carbon composites. Prior to AFRL, he was affiliated with the University of Dayton Research Institute (UDRI) for 10 years.

His current research activities include multifunctional materials, laser–material interactions, strain-resilient electronics, energy transport in nanomaterials, behavior and failure mechanisms in nanomaterials and hybrid graphitic (carbon) foam, and nanomechanics.

Adv. Mater. 2019, 1805598



concentrated H2SO4 and H3PO4 in a controlled chemical process followed by elongated H2O2 treatment near 0 °C, filtration, cleaning, and drying processes.[36,37] The functional properties of 3D carbon nanostructures depend on the constituting carbon allotrope(s) and their physicochemical states, the fabrication pro-cess, and the defect density and type of doped heteroatom(s), along with the electronic states of those dopants.[38,39]

3. Metal-Free Heteroatom-Doped Carbon

Precious-metal catalysts, particularly noble metals such as iridium, palladium, and platinum and associated alloys, have remained the most attractive electrode materials for different electrocatalytic chemical reactions for energy storage and conversion.[41] However, their low selectivity, limited avail-ability, high cost, poor durability, affinity toward gas poisoning, and related potential adverse impact to environment have so far hindered their industrial scale acceptance.[42] Hence-forth, serious efforts are now pursued to replace them with cost-effective and environmentally friendly alternatives. In this context, nonprecious transition metals (Ti, W, Sn, Zn, Fe, Co,

Mo, and Ni), their carbide, nitride, oxide, carbonitride, and oxynitridecatalysts have been and still being widely investi-gated.[41–48] However, the low intrinsic conductivities of such nonprecious-metal-based catalysts, due to the interfacial com-plexity and thermodynamic instability of its hybrid structures, limit its application domain. Recently, carbon-based catalysts have emerged as promising alternatives and are increasingly being utilized as efficient catalysts due to their low density, high specific surface area, suitable electrical and thermal conductivities, chemical stability, and suitable mechanical properties.[2,3,23,40,48]

Carbon-based heteroatom-doped nanomaterials were first introduced in 2009 which, as metal-free catalysts, are compa-rable to platinum for efficiently catalyzing oxygen reduction reaction (ORR) in fuel cells.[49,50] Thereafter, this new class of carbon-based electrocatalysts has been reported to also be effi-cient in oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and many catalytic reactions,[2,51–54] including catalysis of I−/I3− for dye-sensitized solar cells[2] and CO2 reduc-tion reaction (CO2RR) for numerous applications, namely, biosensing, hydrocarbon-based fuel production,[44] monitoring environmental effects,[52] and even for commodity chemicals

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Figure 1. Carbon allotropes in different dimensions. All images except for graphene, graphene oxide, and pillared graphene: Adapted with permission.[1] Copyright 2015, American Chemical Society. Images for graphene, graphene oxide, and pillared graphene: Reproduced with permission.[32] Copyright 2015, AIP Publishing.



production.[2,53,54] More recently, multidoped heteroatom carbon nanostructured electrode materials have demonstrated excel-lent metal-free multifunctional electrocatalysis for ORR, OER, and HER utilized in rechargeable Zn–air (primarily metal–air) batteries,[48] regenerative fuel cells,[55] and simultaneous ORR, OER, and HER reactions.[39,56,57] Such carbon-based heter-oatom-doped single or multifunctional metal-free electrocata-lysts’ performance is often superior to that of noble metal and transition metal catalysts. Notable phenomena of charge induc-tion/modulation around the heteroatom-doped carbon lattice and transport property modulation often contribute to such attractive performance enhancement[2,49]

In fact, the CC bond is nonpolar, but carbon atoms can form polar bonds with heteroatoms to impose different dipole moments depending on their difference in electronegativity (e.g., electronegativity, ε = 2.50 for C, ε = 3.07 for N) and atomic size from those of the carbon atom (Figure 2a). There-fore, a change in the density of states and charge population is attained on both the carbon atom and heteroatom to impart catalytic activities to various heteroatom-doped graphitic carbon materials.[58–68] The properties of nitrogen-doped sp2 carbon nanomaterials are also altered by the chemical state of nitrogen atom doped onto the carbon lattice. It can be pyridinic, pyrrolic, graphitic, or oxidized N state.[69] Although, it is controversial to comment on their differentiation based on the electrocatalytic activity.[70] However, quantum mechanical calculations and experimental investigations indicate that pyridinic N and graphitic N can be beneficial for electrocatalysis in ORR by facilitating O2 adsorption onto the neighboring carbon atoms. This facilitates the four-electron pathway for ORR.[69,71–73] Additionally, sulfur (S; ε = 2.44) doped carbon catalysts are

studied for ORR applications.[30,48] Codoping and tridoping with various heteroatoms, among N, boron (B, ε = 2.01), S, phosphorus (P, ε = 2.06), fluorine (F, ε = 4.17), and oxygen (O, ε = 3.50), have recently been introduced for fabricating multi-functional metal-free catalysts for electrochemical energy con-version and storage.[55,57]

The natural bond orbital (NBO) population analyzed by den-sity functional theory (DFT) shows that different heteroatoms acquire different amount of charge populations within gra-phene lattice. For example, N and O being negatively charged relative to the adjacent C atom culminate as electron acceptors, whereas F, S, B, and P act as donors because they accumulate positive charge.[74] Figure 2b shows the NBO population and the free energy diagrams for HER. In fact, the codoping, tridoping, and even more than three heteroatoms doping in carbon lattice have often been observed to perform synergistic effects toward electrochemical activities. The electrocatalytic performance of 3D carbon-based electrodes depends on the synthesis and fabrication process, type and amount of heteroatom doping, active surface area, extrinsic conducting properties, interfacial resistive and chemical properties at the junctions, length of dif-fusion pathways, as well as the nature and size of pores and preadsorbed entities therein.[2,3,17,23,30] Table 1 compares the experimentally obtained physical properties of various carbon materials with different dimensions.[3,16,75–91]

4. Synthesis of 3D Carbon Materials and Heteroatom Doping

Various chemical vapor deposition (CVD), physical vapor depo-sition (PVD), chemical processing, pyrolysis, 3D printing, and various assembly-based techniques have been devised to fabri-cate 3D carbon materials (Table 1) for electrochemical energy storage and conversion applications.[92–95] Owing to the high efficiency and mechanical stability for electrochemical energy storage, CNT–graphene-based 3D carbon materials, prepared normally by solution processed assembly,[96–102] layer-by-layer assembly,[103–107] and vacuum filtration,[108–110] have been widely studied. The product of assembly processes is generally of inferior quality that would ultimately affect the conductivity and surface area, though this method is simpler than in situ CVD. In situ growth of graphene–CNT hybrid 3D structures (e.g., pillared graphene in Figure 1) is more complicated, but holds better control of the product quality, including their morphology, density, and desired orientation of the hybrid structures. Therefore, different in situ techniques, such as CVD,[111–125] templated CVD,[17,29,126,127] and chemical unzip-ping,[16,128,129] have recently became very popular. Figure 3 shows CVD methods, with and without a template, for the syn-thesis of 3D carbon structures.[17,29,125,126,128] As compared to other CVD techniques, in the templated CVD method particular structural designing is easily achieved. However, comparatively this technique often comes out to be a multistep process, com-plex and expensive. Mass production of porous 3D carbon electrodes can easily be produced through chemical processes although they are inferior in terms of selectively introducing chemical contents, conducting properties and mechanical integrity. Therefore, a high-temperature postpyrolysis step is

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Figure 2. a) Comparison of electronegativity for popular heteroatoms doped in carbon nanomaterials for electrochemical energy storage with their relative atomic size. b) Analysis of natural bond orbital population for six nonmetallic heteroatoms doped in graphene lattice (pN: pyridinic; gN: graphitic N). Inset: The predicted doping location for various elements. Site 1: edge; site 2: in-plane center; site 3: out-of-plane center on gra-phene. b) Reproduced with permission.[74] Copyright 2014, American Chemical Society.



normally applied to reduce chemical impurity and increase conductive properties.[16,23] Besides, carbon aerogels, xerogels, and hydrogels have also been fabricated via chemical routes as low density efficient electrodes.[16] In this context, it would be important to mention that a template-assisted chemical process followed by postpyrolysis step may be interesting to provide particular hierarchy in 3D carbon electrodes.

Recently, 3D carbon electrodes, including MOF-derived mesoporous carbon structures and carbide-derived carbon and onion-like carbon structures,[73,130] have been prepared by solu-tion-based chemical processes, condensation, and coordina-tion reactions. Other major printing techniques, such as inkjet printing, screen printing, and transfer printing, have also been commonly used for depositing nanostructured carbons onto substrates of varying size, surface energy, and flexibility for energy applications, though 3D printing is an emerging tech-nology.[131–141] 3D printing can offer porous carbon structures of 10 µm thick or thicker with very quick drying time (few seconds).[130–134] Carbon allotropic nanomaterials combined with various polymers have been used as highly viscous inks for fab-ricating 3D-printed carbon structures for Li-ion batteries, super-capacitors, water splitting electrolyzers, fuel cells, and envi-ronmental protection applications.[133–137] Figure 4 illustrates various electrodes fabricated by different 3D printing methods.

Heteroatom doping into (3D) carbon nanomaterials can be performed in two ways: during synthesis and postsynthesis. For in situ heteroatom doping during the synthesis of carbon materials, various heteroatom-containing chemical reagents are mixed with carbon sources as precursors or by synthesizing in different heteroatom-containing gas environments. Postsyn-thesis heteroatom doping can be obtained through numerous chemical reactions, and pyrolysis of precursors in the presence of various chemical reagents and gases or by plasma treatments. The carbon structures thus obtained are often processed through activation steps, either by heating with KOH or H2SO4 solution (at a low-temperature (≈100–200 °C) chemical activation) or pyrolysis at a high temperature (≈700–1000 °C) with gases, such as HN3, N2, H2, and CO2 (called gas activation).[3,16] Sometimes some low carbon-containing precursors are carbonized or gra-phitized to maximize its carbon constituents or crystallinity and then it is activated in inert or gaseous environments. This whole process is called physical activation of carbon materials. Het-eroatom-doped carbon nanostructures are generally obtained through this method from various polymers or petroleum prod-ucts.[2,3,16] 3D printing of isotropic heteroatom-doped carbon nanostructures can also be very interesting to conceive.

The chemical activation temperature is generally lower than in physical activation process. Therefore, the probability of

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Table 1. Properties of different carbon allotropes.

Material (dimension) Synthesis method BET surface areaa) [m2 g−1]

Thermal conductivity [W−1 m−1 k−1]

Electrical conductivity [S cm−1]

Ref.

Fullerene (0D) Arc discharge 80–90 0.4 10−10 [75]

Fullerene shoot (0D) Arc discharge of carbon rod 175 – – [76]

Fullerene shoot–CO2 or Ar pyrolyzed (0D) Arc discharge + gas pyrolysis 345–346 – – [76]

Fullerene shoot–KOH activated (0D) Depleted and KOH activated 2153 – – [77]

Nanodiamond (0D) Microwave plasma CVD ≈400–600 1–12 100–10−10 [78–80]

SWCNT (1D) CVD ≈1300 2000–6000 – [75,78,81]

MWCNT (1D) CVD + purify ≈450 3000–3500 – [78,81,82]

MWCNT–KOH activated (1D) CVD + chemical activation ≈785 – – [16]

Graphene or G (2D) CVD ≈1500 5000 ≈2000 to 106 [3,75]

Graphene oxide (2D) Chemical reduction of graphite ≈423–3100 – – [16,83]

Graphite (3D) Natural or synthetic ≈10–20 ≈1500–2000 and

5–10 (cross-plane)

2–3 × 104 and

6 (cross-plane)

[75]

Diamond (3D) Natural or synthetic 20–160 900–2320 10−11 to 10−18 (insulator) [84,85]

GO+CNT (3D) Agglomeration and chemical reduction ≈435 – – [83]

Mesoporous C from rGO (3D) Microwave irradiation+ KOH activation ≈3290 – – [16]

N-doped macro/mesoporous C (3D) Macro/mesoporous silica as a hard template

associated with furfuryl alcohol≈1826 – – [87]

Hydrogel with B-and N-codoped porous

C networks (3D)

Agarose hydrogel pyrolysis containing TBE

buffer with KOH activation≈2666 – – [88]

N/P/O–graphene (3G) Mixture of artemia cyst shells and red P ball

milled: KOH activation/pyrolysis at 850 °C≈1406 – – [89]

N–C microtube sponge (flexible) (3D) Facial cotton pyrolysis at 1000 °C

under NH3 for 1 h

≈2358 – – [90]

P/S C3N4 sponge sandwiched with C

nanocrystals (3D)Pyrolysis at 500 °C followed by freeze-drying ≈1474 – – [91]

a)Detailed atomic representation in Brunauer–Emmett–Teller (BET) surface area is available in cited respective references.



the formation of porous architectures is higher in the chem-ical activation process although proper gas bubbling strategy during the physical activation process can produce micro-porous or sometimes mesoporous materials with high graphitic crystallinity.[3,11] Frankly speaking, the chemical activation pro-cess needs a product postcleaning step to reduce the residual chemicals from the final carbon product.[11,36,37] The specific surface area of the heteroatom-doped carbon structures and their catalytic properties are also dependent on the activation process parameters used. However, more research is required

to confidently comment on such controversial issues. N-doped carbon nanostructured materials have been widely used for electrochemical catalytic applications.[2,3] Besides N and other single heteroatom doping, such as O, B, S, P, and different halogens (i.e., F, Cl, Br, and I), the codoping of NS, BN, NP, NF, PS, and tridoping (such as NPO, etc.) into carbon materials has been performed as discussed in detail in the following sec-tions. The density function theory indicates that the heteroatom doping into carbon lattice modulates the charge distribution characteristics to the adjacent C atom and this modifies the

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Figure 3. a) Schematic for synthesis of aligned CNT/graphene sandwiches on bifunctional catalysts graphene sandwiches, b) digital photograph of a block of EV catalyst (brown) and aligned CNT/graphene sandwich and c,d) SEM of aligned CNT/graphene/EV composite. a–d) Reproduced with permission.[125] Copyright 2014, Elsevier Inc. e) Schematic of 3D pillared VACNT–graphene architecture. f) Detailed procedure for fabrication of 3D pillared VACNT–graphene structure. g,h) Digital images of HOPG with 80 µm thickness (g) and thermally expanded VACNT intercalated graphene layers (h). i–k) SEM of 3D pillared VACNT–graphene nanostructures. e–k) Reproduced with permission.[126] Copyright 2013, American Chemical Society. l,m) Schematic of synthesis procedure and structural details of 3D graphene–RACNT fiber. n,o) SEM and AFM images of the 3D graphene–RACNT fiber, and p) its SEM and EDX elemental map-ping. l–p) Reproduced with permission.[29] Copyright 2015, American Association for the Advancement of Science. q) Schematic formation of the GONR/CNT hybrid, and r,s) its TEM and SEM images. q–s) Reproduced with permission.[128] Copyright 2013, Wiley-VCH. t–v) SEM of 3D CNT foam obtained by growing VACNTs on Ni foam template by MPCVD, inset of (t) shows bare Ni foam. t–v) Reproduced with permission.[17] Copyright 2016, Royal Society of Chemistry.



related electrocatalytic performance. Therefore, different het-eroatom/heteroatom incorporation relates to modification in electrochemical performance in different degrees for different catalytic reactions (such as ORR, OER, and HER). Codoping and tridoping have been established by numerous studies to be beneficial for such electrocatalytic applications which are often overlooked as synergistic effects. Therefore, more detailed study is required to clearly understand the mechanism of multidoping effect on electrochemical catalysis.

5. Mesoporous 3D Carbon as Catalysts for Oxygen Reduction Reaction

For the biological respiration process, ORR is a crucial reaction and it is also technologically important in energy-converting systems, such as metal–air batteries and fuel cells, etc.[3,30,67] There are different types of fuel cells. As compared to other fuel cells, solid-oxide-based high-temperature fuel cells (SOFCs), polymer-electrolyte-based membrane fuel cells (such

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Figure 4. a) Schematic of fabrication steps for 3D-printed graphene composite aerogels (3D-GCA). Reproduced with permission.[135] Copyright 2016, American Chemical Society. b) Digital image and c–e) SEM images of 3D-printed microlattice using graphene aerogel (c), and graphene aerogel without (d) and with 4 wt% R–F (e) after etching. b–e) Reproduced with permission.[136] Copyright 2015, Macmillan Publishers Limited. f) Step I: self-assembled 3D graphene hydrogel (GH) from GO sheets; Step II: in situ polymerization of PANI onto GH; Step III: ball milling and ultrasonic exposure of 3D GH–PANI composite for GH–PANI ink preparation; Step IV: GOP formation by printing GO ink on paper; Step V: overprinting of GH–PANI inks onto GOP; Step VI: soaking of GH-PANI/GOP in diluted HI; Step VII: simultaneous reduction of GH–PANI/GOP using HI and forma-tion of freestanding GH-PANI/GP after peeling off; Step VIII: fabrication of flexible supercapacitor (SC) device using gel electrolyte onto GH-PANI/GP electrode. Reproduced with permission.[137] Copyright 2014, American Chemical Society.



as proton-exchange-membrane fuel cells, PEMFCs), and direct alcohol-based fuel cells (for example, direct methanol fuel cell, DMFC) are very well known for having lower tempera-ture of operation and longer cell life. These fuel cells are very promising for futuristic lightweight electric vehicles as well as portable electronics.[142] In both PEMFC and DMFC, precious Pt-based catalyst, the key component, is mounted together with a mesoporous conducting material (such as carbon black) either as anode or cathode, for electrocatalysis of hydrogen (named as hydrogen oxidation reaction, HOR) and alcohol (termed as alcohol oxidation reaction, AOR) or oxygen (called oxygen reduction reaction), distinctively. However, the lethargic reaction kinetics for HOR and AOR at relatively low operating temperature of 80–100 °C as well as high energy input neces-sary for ORR induces a large polarization resistance.

ORR occurs in multisteps related either to a four-electron (4eˉ) path for direct H2O generation in acidic electrolyte and OHˉ production in alkaline electrolyte or a two-electron (2e−) process forming H2O2 in acidic electrolyte and HO2ˉ emission in alkaline electrolyte as intermediate products. Such reac-tion pathway mainly depends on the inherent electrochemical catalytic characteristics of the electrodes. The usual reaction mechanisms in the presence of different electrodes and media of varied thermodynamic potential, operating under standard condition, is presented below:[143,144]

ORR in acidic electrolyte

O 2H 2e H O 0.7 V, a two electron process2 2 2 ( )+ + → −+ −

O 4H 4e 2H O 1.229 V, a four electron process2 2 ( )+ + → −+ −

ORR in alkaline electrolyte

O H O 2e HO OH 0.065 V, two electron process2 2 2 ( )+ + → + − −− − −

H O HO 2e 3OH 0.867 V2 2 ( )+ + →− − −

O 2H O 4e 4OH 0.401 V, a four electron process2 2 ( )+ + → −− −

For a given electrocatalyst, a selection of higher 4eˉ reduction pathway facilitates a superior ORR electrocatalysis.

Nanoparticulated Pt catalyst is considered as the best electrocatalyst for the ORR since long past, although with con-cerns of high related cost and scarcity which have prejudiced the fuel cells from industrialization. With an aim to reduce or even completely replace Pt-based catalysts with alternatives with lower cost, there have been tremendous research studies. We had previously discovered that heteroatom-doped graphitic carbon nanomaterials (for example, CNTs, graphene) could act as metal-free ORR catalysts with reduced cost and comparable efficiency in fuel cells.[2,3,49,145] Among heteroatom-doped 3D carbon catalysts, nitrogen doping in varied carbon architectures, including MWCNT–rGO hybrids, rGO-based aerogels, xerogels, hydrogels, porous carbon (meso- and macroporous), graphene foams, carbon capsules, polymer, and MOF-derived carbon net-works, has been reported in the literature.[58–64,66,67,84,87,146–171] Carbon nitride (C3N4)-based carbon composites,[67,73,169–171]

graphene foams,[172–174] and S-doped carbon aerogel have also been extensively studied for ORR applications. Furthermore, S/N,[175–185] B/N,[88,186–188] P/N[189–192] and F/N[193,194] codoping and N/P/O[119] tridoping have recently been achieved to demon-strate the bifunctional and trifunctional carbon-based electro-catalysis for energy storage and conversion.

Nitrogen-doped porous 3D carbon materials are very well known for their high ORR activity and exceptional conductivity, along with good retentivity. For instance, N-doped mesoporous graphitic stacks (arrays) showed high electrocatalytic activity, stability, and methanol tolerance in alkaline solutions.[60] Fur-thermore, an aerogel based on N-doped graphene nanoribbons (GNRs) acted superb ORR electrocatalytic performance and superior stability compared to commercial Pt/C in alkaline as well as acidic media.[61] In another effort, N-doped carbon nanofiber (N-CNF) exhibited high N content (5.8 at%), high active-site density related to N-doping, and high BET surface area about 916 m2 g−1.[157] This N-CNF-based aerogel has also exhibited excellent ORR activity with 0.85 V onset potential versus reversible hydrogen electrode (RHE), high (3.97 at 0.8 V) electron-transfer number, and attractive electrochemical ORR stability of about 97.5% retention even at 10 000 potential cycles (0.1 m KOH).[157] This N-CNF aerogel has also demonstrated a 0.83 V onset potential in 0.5 m H2SO4 media,[157] which is very promising performance worth mentioning.

Hierarchically N-doped cross-linked porous 3D carbons called LHNHPC structure possess a specific surface area of 2600 m2 g−1, 3.12 at% N-dopants, and an ORR activity closely comparable to Pt/C catalyst, the benchmark in basic media.[84] Such excellent electrochemical properties have often been attributed to the synergistic effects arising due to the mesoporous structure, doped N-heteroatoms, and the high specific surface area.[84]

Table 2 summarizes important reports on various N-doped and S-doped 3D carbon materials for ORR application. Among them, it is worth to note that S-doped 3D aerogel and graphene foam showed superior ORR performance to their undoped carbon structures (Table 2),[172,173] although sulfur has identical electronegativity as of carbon implying that the doping-induced charge-transfer remains neutral. However, the hydrophobic nature of the thiophene-like form of S within the carbon lat-tice plays a crucial role in drawing oxygen out of the electrolyte that facilitates its physical adsorption on the electrode surface, primarily enhances the ORR mechanics.[73,173] Other bulky oxygen-containing sulfur surface groups (e.g., sulfonic acids, sulfoxide, sulfones) situated in larger pores have also been dem-onstrated to attract electrolyte solutions containing dissolved oxygen toward the pore system.[174] However, possible contribu-tions to the enhanced ORR performance by the sulfur-doping induced spin redistribution cannot be ruled out.[175] Therefore, further more research is needed to explore the S-doping effect on ORR activities in 3D hierarchically porous carbon materials.

N- and S-codoped hollow 3D carbon-sphere (porous) nano-composites (N,S-hcs) demonstrated excellent ORR electrocata-lytic performance (Figure 5a–e) with a four-electron transfer pathway, appreciable durability, and high tolerance to methanol crossover/CO-poisoning effects.[180] This work demonstrates the synergistic effect associated with heteroatom codoping into porous C structures.[181]

Adv. Mater. 2019, 1805598



Adv. Mater. 2019, 1805598

Tabl

e 2.

Nitr

ogen

- or

sulfu

r-do

ped

3D p

orou

s ca

rbon

cat

alys

ts fo

r O

RR

.

Mat

eria

lSy

nthe

sis

met

hod

Che

mic

al c

onte

nt a

nd p

rope

rty

Elec

trol

yte

Ons

et p

oten

tial

[V v

s R

HE]

and

nTa

fel s

lope

[m

V p

er d

ecad

e]St

abili

tyR

ef.

MW

CN

T–rG

O h

ybri

dPo

ly(d

ially

ldim

ethy

lam

mon

ium

chl

orid

e

func

tiona

lized

hyd

roph

ilic

MW

NTs

and

rG

O

elec

tros

tatic

sel

f-ass

embl

y

rGO

:pM

WN

T ≈

1:2;

the

char

ge/g

: 2.6

6

C/g

for

rGO

, 2.1

6 C

/g fo

r pM

WC

NTs

and

62.9

C/g

for

rGO

/pM

WC

NT

(0.5

:1)

0.1

m K

OH

0.8

V, 3

.6–3

.361

% a

fter

20

000

s[1

46]

rGO

/act

ivat

ed c

arbo

n

aero

gel

GO

+ ac

tivat

ed c

arbo

n+ a

mm

oniu

m h

ydro

xide

,

auto

clav

ed (

12 h

), ly

ophi

lized

(12

h)

SA ≈

758

.19

m2

g−10.

1 m

KO

H0.

83 V

, 3.5

2–

[147

]

N–G

flow

erC

o-py

roly

sis

(500

–700

°C)

of m

elam

ine

and

ferr

ocen

eN

-3.6

2 at

% (

600

°C)

0.1

m N

aOH

0.87

V, 3

.96

[148

]

N–g

raph

ene

foam

Pyro

lysi

s-90

0 °C

of s

ilica

sph

eres

, FeC

l 2 ⋅4

H2O

,

dicy

andi

amid

e an

d G

O.

N-5

.07%

, SA

≈ 6

70 m

2 g−1

0.1

m K

OH

0.1

m H

ClO

4

0.95

V (

Pt/C

-0.9

6), 3

.9

0.83

V, 4

98.3

and

95.

3%

afte

r 50

00 c

ycle

s

[149

]

Hie

rarc

hica

l NG

/por

ous

C

com

posi

teG

O/e

thyl

ened

iam

ine,

950

°CSA

≈ 1

510.

83 m

2 g−1

0.1

m K

OH

0.91

V, 3

.79

70 m

V p

er d

ecad

e

(68

for

Pt/C

)

18%

aft

er 2

1 60

0 s

[58]

3D N

GPy

roly

sis

of G

O+p

olyp

yrro

le, 9

00 °C

2–3%

, SA

≈ 3

70 m

2 g−1

0.1

m K

OH

0.83

V, 3

.9m

in. a

fter

200

0 cy

cles

[59]

N–G

nan

orib

bon

aero

gel

Che

mic

al u

nzip

ping

of M

WC

NTs

, hyd

roth

erm

al m

ixin

g

with

pyr

role

, pyr

olys

isSA

≈ 4

80–6

17 m

2 g−1

,

dens

ity ≈

2.5

–22

mg

cm−3

0.1

m K

OH

0.81

V, 3

.66–

3.92

78%

aft

er 2

0 00

0 s

[61]

N–C

xer

ogel

Pyro

lysi

s of

org

anic

xer

ogel

in N

H3

and

pyro

lyat

eSA

≈ 5

41 m

2 g−1

0.5

m H

2SO

40.

84 V

, 3.5

–3.7

103

mV

per

dec

ade

(pt/

C-7

2)

[62]

N–C

aer

ogel

Pyro

lysi

s-90

0 °C

, org

anic

aer

ogel

(sy

nthe

size

d fr

om

gluc

ose

and

bora

x)

N: 2

.0–5

.5 w

t%,

SA: ≈

265

m2

g−10.

1 m

KO

H≈0

.81

V, 3

.385

%–2

00 m

in[1

50]

3D N

anop

orou

s N

–GC

VD

G g

row

th o

n na

nopo

rous

Ni f

oam

with

pyri

dine

—10

00 °C

5%0.

1 m

KO

H0.

86 V

, 3.9

[63]

3D fl

ower

-like

N–C

Ferr

ocen

e an

d m

elam

ine:

cop

yrol

ysis

at 5

00–7

00 °C

4.62

%0.

1 m

NaO

H0.

87 V

, 3.9

6[1

48]

N-m

acro

/mes

o-C

Har

d te

mpl

ated

mac

ro/m

esop

orou

s si

lica

in fu

rfur

yl a

lcoh

olSA

≈ 1

826

m2

g−10.

1 m

KO

H0.

87 V

, 3.2

69.6

mV

per

dec

ade

93.8

% a

fter

the

45 0

00 s

[87]

N/C

NTs

nan

otub

es/

carb

on n

anofi

bers

com

-

posi

te (

NC

NT/

CN

Fs)

Pyri

dine

pyr

olys

is o

ver

flexi

ble

elec

tros

pun

carb

on

nano

fiber

s fil

m (

CN

Fs)

with

the

nano

-Fe

cata

lyst

O-7

.9, N

-4.8

at%

0.1

m K

OH

0.91

V, 3

.894

.65%

—10

000

s[1

51]

Nitr

ogen

-dop

ed c

arbo

n

foam

(C

FN)

Com

bust

ion

of d

ieth

anol

amin

e (N

) an

d et

hano

l

(C s

ol–g

el)

with

res

orci

nol (

R)

and

form

alde

hyde

-

pyro

lysi

s in

am

mon

ia—

post

heat

ing

at 8

00 °C

0.65

at%

N0.

1 m

KO

H0.

92 V

, 3.9

94.7

% a

fter

60

000

cycl

es[1

52]

N–C

xer

ogel

5 at

% N

0.1

m N

aOH

0.96

V, 3

.797

.7%

aft

er 1

000

cycl

es[1

53]

N–G

foam

sSi

lica

sphe

res+

GO

+ N

(cy

anam

ide,

mel

amin

e an

d ur

ea)-

900

°CN

-3.1

5 at

%, S

A ≈

918

.7 m

2 g−1

0.1

m K

OH

0.1

m

HC

lO4

0.94

, 3.9

0.81

, 3.8

≈97%

aft

er 5

000

cycl

es[1

54]

3D N

–GG

O in

eth

anol

+3-a

min

opro

pyltr

ieth

oxys

ilane

+90

0 °C

2.92

%, 9

13 m

2 g−1

0.1

m K

OH

0.93

V, 3

.710

0% a

fter

800

s[1

55]

N-m

acro

/mes

opor

ous

CA

utoc

lave

of m

elam

ine,

res

orci

nol,

hexa

met

hyle

nete

tra-

min

e, a

nd c

ollo

idal

sili

ca-(

120

°C)-

pyro

lysi

s 60

0–90

0 °C

3.48

%, 8

45 m

2 g−1

0.1

m K

OH

0.91

V, 4

Smal

l cha

nge

afte

r 40

00 c

ycle

s

[156

]

N–C

nan

ofibe

r ae

roge

lPy

roly

ze-d

ry b

acte

rial

cel

lulo

se d

eriv

ed a

erog

el in

N2

at

800

°C C

NF

aero

gels

. hea

ting

in N

H3

at 7

00–9

00 °C

5.8%

, SA

≈ 9

16 m

2 g−1

0.1

m K

OH

0.5

m

H2S

O4

0.85

V, 3

.96

0.83

V,

97.5

% a

fter

10 0

00 c

ycle

s

[157

]



Adv. Mater. 2019, 1805598

Mat

eria

lSy

nthe

sis

met

hod

Che

mic

al c

onte

nt a

nd p

rope

rty

Elec

trol

yte

Ons

et p

oten

tial

[V v

s R

HE]

and

nTa

fel s

lope

[m

V p

er d

ecad

e]St

abili

tyR

ef.

N–C

nan

ofibe

r ae

roge

lsH

ydro

ther

mal

car

boni

zatio

n—ul

trat

hin

tellu

rium

nano

wir

e te

mpl

ated

glu

cosa

min

e hy

droc

hlor

ide

at 9

00 °C

; CO

2 ac

tivat

ion

at 1

000

°C

N-1

.64

at%

, 132

4 m

2 g−1

0.1

m K

OH

0.93

V, 3

.74–

4.02

87.2

% a

fter

15

000

s[1

58]

N–C

cap

sule

sG

lyox

al d

ispe

rsed

in e

than

ol a

nd m

elam

ine,

pyro

lysi

s—80

0 °C

N/C

-15,

SA

≈ 5

95 m

2 g−1

0.1

m K

OH

0.91

V, 3

.780

% a

fter

252

000

s[1

59]

N-h

oley

gra

phen

eG

O tr

eate

d in

nitr

ic a

cid

(con

c.);

then

sol

voth

erm

ally

trea

ted

with

ure

aN

-8.6

at%

, SA

≈ 7

84 m

2 g−1

0.1

m K

OH

0.88

5 V,

3.8

590

% a

fter

10

000

s[1

60]

Poly

mer

der

ived

3D

N–C

netw

ork

Self-

asse

mbl

ed p

olya

nilin

e in

side

NaC

l via

rec

ryst

al-

lizat

ion,

pyr

olys

isN

/C ≈

3.5

2 to

3.8

5%, S

A ≈

265

.7 m

2 g−1

0.1

m K

OH

0.78

V, 3

.9[6

4]

Poro

us N

–Co-

Phen

ylen

edia

min

e an

d si

lica

collo

id

pyro

lysi

s at

900

°C in

Ar

N ≈

3.1

5 to

9.5

at%

, SA

≈ 1

280

m2

g−10.

1 m

KO

H≈0

.92

V, 2

.62

to 3

.86

≈98%

aft

er 1

0 00

0 cy

cles

[161

]

Hie

rarc

hica

l N–C

Sodi

um a

lgin

ate

and

urea

pyr

olys

isSA

≈ 4

70.9

m2

g−1, N

≈ 2

–8 a

t%0.

1 m

KO

H0.

94 V

, ≈4

68 m

V p

er d

ecad

e81

% a

fter

20

000

cycl

es[1

62]

MO

F-de

rive

d N

–CM

OF

pyro

lysi

s an

d N

H3

activ

atio

nN

≈ 4

.4–5

.5 a

t%, S

A ≈

241

2 m

2 g−1

,

D ≈

6.8

nm

0.1

m K

OH

0.5

m H

2SO

4

0.96

V, 3

.9–4

0.84

V,

53 m

V p

er d

ecad

e

98 m

V p

er d

ecad

e

98.3

3% a

fter

10 0

00 c

ycle

s-

[163

]

MO

F-de

rive

d, p

orou

s N

–CPy

roly

sis

of M

g-M

OFs

at 1

000

°C—

1 h

N-1

.3 to

5.7

at%

, SA

≈ 1

500

m2

g−10.

1 m

KO

H0.

82–0

.915

V, 3

.6–3

.9[1

64]

MO

F-de

rive

d

pyri

dini

c N

–CA

utoc

lave

(14

0 °C

)—2,

2′-b

ipyr

idin

e-5,

5′-d

icar

boxy

late

,

AlC

l 3⋅6

H2O

and

ace

tic a

cid;

pyr

olyz

ed a

t 100

°C—

12 h

N ≈

7.6

at%

, SA

118

0 m

2 g−1

0.1

m K

OH

0.89

V, 3

.82

95%

aft

er 9

000

s[1

65]

N-C

NT/

rGO

from

pol

ymer

Adh

esio

n of

pol

y(p-

phen

ylen

evin

ylen

e)

(PPV

) pr

ecur

sor

N-4

.21

at%

, SA

≈ 2

68 m

2 g−1

0.1

m K

OH

0.92

V, 3

.99

92%

aft

er 4

0 00

0 s

[166

]

Gra

phen

e sh

eets

insi

de

CN

T vo

ids

Ann

ealin

g

FeC

l 3 a

nd c

yana

mid

e m

ixtu

re—

900

°C in

Ar

N-6

.6 a

t%, S

A ≈

363

.8 m

2 g−1

0.1

m K

OH

0.88

5 V,

3.9

8–

[167

]

N-p

orou

s ca

rbon

Pyro

lysi

s (1

000

°C)

Res

orci

nol +

form

alde

hyde

+

amm

oniu

m c

arbo

nate

-pyr

olys

is in

NH

3 flo

w

N-3

.12

at%

, SA

≈ 2

986

m2

g−10.

1 m

KO

H0.

95 V

, 3.7

598

.6%

aft

er 1

0 00

0 cy

cles

[84]

N–C

Poly

mer

izat

ion:

mel

amin

e–re

sorc

inol

–

hexa

met

hyle

nete

tram

ine,

pyr

olys

is a

t 900

°C in

N2

N-3

.48

at%

, SA

≈ 8

45 m

2 g−1

0.1

m K

OH

0.91

V, ≈

498

.7%

aft

er 4

000

cycl

es[1

56]

N–C

(co

al ta

r de

rive

d)C

oal t

ar p

roce

ssin

g an

d ox

idiz

ing

in c

once

ntra

ted

acid

s an

d py

roly

zed

at 9

00 °C

N-3

.5 w

t%, S

A ≈

198

5.1

m2

g−10.

1 m

KO

H0.

92 V

, >3.

8 (0

.27–

0.85

V)

98.8

% a

fter

300

0 cy

cles

[168

]

Spon

ge-li

ke N

–C/

grap

hitic

-C3N

4

Pyro

lysi

s of

C-c

hitin

/g-C

3N4

solu

tion

with

NaO

H a

t

800

°CN

-13.

19 a

t%0.

1 m

KO

H0.

89 V

, 3.9

76 m

V p

er d

ecad

e86

.8%

aft

er 5

0 00

0 s

[169

]

Mes

opor

ous

C3N

4/C

arbo

nPy

roly

sis:

sili

ca m

icro

sphe

res

with

suc

rose

-900

°C,

cyna

mis

e at

550

°CC

3N4

0.1

m K

OH

0.84

V, 3

150,

190

mV

per

deca

de≈8

5% a

fter

60

h[1

70]

Poro

us g

raph

itic

C3N

4/

carb

on c

ompo

site

sph

eres

Pyro

lysi

s (6

00 °C

) of

mel

amin

e an

d cy

anur

ic a

cid,

and

gluc

ose

mix

ed

N/C

≈ 0

.69,

SA

≈ 4

50 m

2 g−1

0.1

m K

OH

0.9

V, 3

.480

% a

fter

20

000

s[1

71]

C3N

4 na

nosh

eets

and

rG

O

self-

asse

mbl

y

Pyro

lysi

s—m

elam

ine:

g-C

3N4,

mix

ture

of g

-C3N

4

and

GO

with

xen

on la

mp

in m

etha

nol

SA ≈

310

m2

g−10.

1 m

KO

H0.

85 V

, 3.8

93%

aft

er 2

0 00

0 s

[73]

S–C

aer

ogel

/GO

com

posi

te

C-a

erog

el:e

sorc

inol

–for

mal

dehy

de p

olym

er a

erog

el,

S–C

aer

ogel

: C a

erog

el h

eatin

g at

650

°C a

nd 8

00 °C

in

H2S

for

3 h,

GO

and

aer

ogel

son

icat

ion

S-2.

5%, 4

12 m

2 g−1

0.1

m K

OH

0.82

9 V,

2–3

.970

% a

fter

130

0 cy

cles

[172

]

S–gr

aphe

ne fo

amSo

lvot

herm

al w

ith G

O a

nd N

a 2S

S-0.

5%, S

A ≈

66

m2

g−10.

1 m

KO

H–

–≈1

00%

—10

00 c

ycle

s[1

73]

Tabl

e 2.

Con

tinue

d.



Apart from N/S-hcs, various other codoped 3D carbon nano-structures, including N/P-, B/N-, N/F-, and P/C3N4-codoped hierarchal carbon architectures, have also been fabricated to achieve superior ORR electrocatalytic performance as com-pared to Pt/C, as summarized in Table 3. Furthermore, three elemental doping (i.e., tridoped) in 3D porous carbon structure has also been studied,[30,57] though still not much discussed in the literature as compared with codoping. In particular, N–P–O-tridoped freestanding 3D graphene foam (3D-HPG) has almost isotropic feature, having pores ranging from 50 nm to 1 µm, and exhibited superior ORR catalytic performance of 92% cur-rent density retention in alkaline electrolyte with 0.93 V onset potential (vs 81% retention for Pt/C) even after 10 000 s of reaction.[89] Density functional theory calculations indicated that N–P–O tridoping is an attractive option for electrocatalysis since it could significantly enhance the charge delocalization.

It should be mentioned that structural defects, even without heteroatom doping, have recently recognized to also impart cat-alytic activities to pure carbon nanomaterials via the so-called defective doping–induced charge redistribution.[195–199] Uti-lizing this concept, graphene nanoribbon supported graphene quantum dots were developed showing ORR performance very closely comparable or superior than the state-of-the-art Pt/C electrode.[200] This electrocatalytic excellence may be attributed to the surficial, interfacial, and edge defect sites of quantum dots and graphene nanoribbons, as well as to the efficient charge transfer through their intimate interfaces. Clearly, the quantum defect and interface modification can also be adopted to 3D porous carbon structures with or without heteroatom doping to develop defect-induced ORR electrocatalysis. In this context, biomass-derived 3D porous carbon materials (e.g., chitin, keratin, and shrimp shell)[201–204] and d-glucose[205] components are also reported to demonstrate as desirable ORR catalytic performance in microbial fuel cells.[206]

Clearly, heteroatom-doped 3D carbon structures are supe-rior electrodes for ORR performance due to increase in charge redistribution aided increase in O2 adsorption and highly abundant active sites in three dimensions. N-doped 3D carbon nanostructured materials have been widely used with a few reports on S-doped carbon structures for ORR applications. Heteroatom-codoped 3D structures are superior to single-doped counterparts as evidenced by many studies. Among them, NS- and NP-codoped 3D mesoporous carbon materials are most effi-cient for electrocatalysis of ORR and possess potential to replace precious-metal-based catalysts. However, more research is required to clearly understand the influence of structural details of 3D carbon nanostructures on ORR performance. There needs to bring more advancement in acid-electrolyte-based 3D carbon catalysts for commercial success in fuel cell technology.

6. 3D Carbon-Based Electrocatalysts for Oxygen Evolution Reaction

Multiple proton–electron-coupled steps that are involved in oxygen evolution reaction to produce molecular oxygen[207,208] is pH sen-sitive. In basic solution, hydroxyl groups (OH−) are oxidized into H2O and O2, whereas in acidic and neutral electrolytes, two adja-cent water molecules (H2O) are oxidized in four protons (H+) and oxygen molecule (O2). The equilibrium half-cell potentials ( a

0E ) at 1 atm and 25 °C for OER are shown as follows:[209]

OER in alkaline solution

4OH 2H O O 4e 0.404 V2 2 a0E↔ + + =−

OER in acidic solution

2H O 4H O 4e 1.23 V2 2 a0E↔ + + =+

Adv. Mater. 2019, 1805598

Figure 5. a) Schematic for N/S-hcs preparation, b) LSVs of N,S-hcs treated at various temperatures, and c) samples (at 1600 rpm and 5 mV s−1). d) LSVs of N,S-hcs-900 °C are at different rotating speeds and e) the Koutecky–Levich curves at different potentials. a–e) Reproduced with permission.[180] Copyright 2016, The Royal Society of Chemistry.



Adv. Mater. 2019, 1805598

Table 3. Doped (co- and tridoped) carbon-based 3D catalysts for ORR.

Material Synthesis method Chemical content and property

Electrolyte Onset potential [V] and n

Tafel slope [mV per decade]

Stability Ref.

N/S-codoped C

N/S-G frameworks Graphene oxide and ammonium

thiocyanate/autoclave (180 °C)

N, S 12.3%

and 18.4%

0.1 m KOH 0.27

(Ag/AgCl, 3 m), 3.9

85.2%

after 20 000 s

[176]

N- and S-codoped

micro/mesoporous

carbon foams

Templateless and no toxic gas

thermal reaction at high temperature

(1000 °C) between sulfur sphere core

and polyaniline shell

N, S: ≈0.58, 1 at% 0.1 m KOH 0.051 V, 3.67 76.5% after

20 000 s

[177]

N/S C with

macroporosityMonodispersed silica spheres+ 1-methyl-

3-propagylimidazolium bromide]

[bis(trifluoromethyl)sulfonyl imide+ pyrolysis

in N2 at different temps, SiO2 remove in HF

4.4%, 2.6%, 1.1% and

1.0% at 800, 900,1000,

and 1100 °C

SA ≈ 1146.6 m2 g−1

(1100 °C)

0.1 m KOH −0.11 (Hg/HgO),

3.4 to 3.8 in −0.3 to

−0.7 V range

95% after 10 000 s [178]

Porous S/N–C

from honeysucklesHoneysuckles pyrolysis in N2 at 600–900 °C S-0.46%, N-1.75,

SA ≈ 246 m2 g−1

0.1 m KOH 0.027 (Hg/HgO), 3.6 Good stability after

1000 cycles

[179]

N/S-doped hollow

C sphere

Triton X-100, pyrrole, aniline,

ammonium persulfate: pyrolysisN ≈ 4.2% (wt),

S ≈ 3.8% (wt)

0.1 m KOH 0.93 V, 3.9 90% after 20 000 s [180]

S/N–C aerogel Glucose and ovalbumin heated in autoclave

at 180 °C for 5.5 h; N source: 900 °C pyrol-

ysis in N2; S source: S-(2-thienyl)-l-cysteine

(TC) or 2-thienyl-carboxaldehyde (TCA),

N-3.3%, S-0.5%.

SA ≈ 224.5 m2 g−1

0.1 m KOH

0.1 m HClO4

−0.13 V

(Ag/AgCl), 2–4

>0.2 V

(Ag/AgCl), 2–4

89% and

≈75% after 12 000 s

[182]

N/S-porous carbon iron and polyquaternium-2 dispersed onto

SiO2 spheres followed by coagulation,

pyrolysis, SiO2 removal and H2SO4 leaching

N ≈ 3.9% (at), S ≈ 0.76%

(at); SA ≈ 1201 m2 g−1,

pore, d ≈ 1–28 nm

0.1 m KOH

0.5 m H2SO4

0.95 (SHE), 3.97

0.8 (SHE), 3.97–3.99

100% and

95.2% after

5000 cycles

[183]

N/S graphene Self-assembled pyrrole polymerized on

graphene oxide template, pyrolysis≈9.05% (at),

S ≈ 1.65% (at)

0.1 m KOH

0.5 m H2SO4

0.338 (Ag/AgCl), 4

0.73 V, 3.92

78.2, 77.4%

after 30000 s

[184]

N/S-porous C/Pt Ionic liquid (1 butyl 3 methylimidazolium

bis(trifluoro methylsulfonyl)imide)

(C10H15F6N3O4S2)+KCl+ZnCl2

pyrolysis at 850 °C

N, S ≈ 7.03, 1.68 at%,

SA ≈ 1424 m2 g−1

0.5 m H2SO4 0.85 (SCE), 4 ≈100% after

126 000 s

[185]

N/S-C nanowire

aerogel

Hydrothermal synthesis using

d-(+)-glucosamine hydrochloride

and thiourea, pyrolysis at 900 °C

N-6.38%, S-0.84 at%,

SA ≈ 870 m2 g−1

0.1 m KOH 0.905 V, 4.3 91% after 20 000 s [186]

B/N-codoped C

N-, B-, B/N-doped

3D G networks

Ni foam template and

electrochemical doping

0.1 m KOH −3.7 (Ag/AgCl), 3.6

and 3.8, respectively.

85%, 70% and

82% after 3000 s

[187]

Hydrogel: B/N-

doped porous C

network

Pyrolysis of agarose hydrogel containing TBE

buffer (boric acid, tris base, and ethylenedi-

aminetetra acetic acid); KOH activation.

B/C ≈ 0.1%,

N/C ≈0.7%,

SA ≈ 2666 m2 g−1

0.1 m KOH −0.18 (Ag/AgCl),

3.64

95.3% after

10 000 cycles

[88]

B/N-doped gra-

phene aerogel

GO treated with urea and chitosan, then the

product pyrolyzed at 1000 °C mixing with

boric acid

Pore diameter ≈

88.3–185.6 nm,

SA ≈ 545.7 m2 g−1,

0.1 m KOH −0.07 (SCE), 3.87 73 mV

per decade

95% after 10 000 s [188]

h-B/N rGO

composite

rGO/BN by hydrothermal method,

annealing at 750 °C in N2

hBN ≈ 5 wt% 0.1 m KOH 0.798 (Ag/AgCl,

Sat.), 3.8≈95% after

10 000 cycles

[189]

N/P-codoped C

N/P porous C copyrolyzing N and P containing precursors

+ poly(vinyl alcohol)/polystyrene: hydrogel

composites

1083 m2 g−1 0.1 m KOH 0.946 (RHE), 3.9–4 [190]

N/P-G

nanoribbons/CNT

composites

Partially unzipped CNTs and ammonium

dihydrogen phosphate (NH4H2PO4)

grounded and annealed 900 °C

for 2 h in N2

N-0.94%

P-2.56%

0.1 m KOH 0.90 (RHE), 3.97 46.7 mV per

decade≈96% after 600 s [191]

N/P-porous carbon Melamine+amino trimethylene phosphonic

acid+carbon quantum dots, pyrolyzed up to

1000 °C in Ar for 1 h

N-3.48, P-0.91 at%, SA

≈ 743 m2 g−1 (900 °C)

0.1 m KOH

0.5 m H2SO4

0.92 (RHE), 3.89

0.74 (RHE), 3.64

84.2%

73.6%

after 12 000 s

[192]



In order to maintain the working voltage around 1.23 V and to avoid the pH influence on the applied potential’, a reversible hydrogen electrode is usually adapted as reference.[209] As it is known, the evolution and transfer of four electrons out of O2 molecule needs multistep reactions of single electron transfer in each step to achieve favorable kinetics for OER process to take place. Also, sluggishness in OER kinetics is triggered by the energy accumulation at each step, resulting in a huge over-potential. So, to overcome the energy barrier highly active OER electrocatalyst is required.[210,211] Consequentially, characteris-tics of an ideal OER catalyst should be of high catalytic stability

with related low overpotential, low cost, and earth-abundance for large-scale commercialization.

The heteroatom-doped 3D carbon materials and their OER catalytic performance is summarized in Table 4. Although N-doped mesoporous carbon showed a better OER catalytic sta-bility,[204] N/O-codoped hydrogel[214] has shown a lower onset potential (1.28 V) for OER in 0.1 m KOH besides that observed in N-doped porous carbon[212] and C3N4/CNT composite cata-lysts.[213] The (0.5 m H2SO4) N/O hydrogel exhibited a better OER performance as compared to that in commercial IrO2 electrode in acid medium.[214] In an alkaline medium of (0.1 m KOH) even

Adv. Mater. 2019, 1805598

Table 4. Carbon-based heteroatom-doped 3D catalysts for OER.

Material Method Content and property

Electrolyte Onset potential [RHE] and n

Potential at 10 mA cm−2 [RHE] and Tafel slope

[mV per decade]

Stability Ref.

N-porous C Pyrolyzed GO and dimethylsulfoxide

mixture at 700 °C

N-4.1 at%,

SA ≈ 560 m2 g−1

0.1 m KOH 1.52 V, 3.9 1.61 V ≈100% after

20 cycles

[212]

C3N4/CNT Low-temperature self-assembled

CNTs and g-C3N4 sheetsSA ≈ 149 m2 g−1,

N-23.7 wt%

0.1 m KOH 1.53 V, 4 1.6 V, 83 86.7% after 10 h [213]

N/O–C hydrogel rGO and CNTs layer-by-layer

assembly, autoclaved N-doping with

ammonia

N/C-9.9%,

SA ≈ 519 m2 g−1

0.1 m KOH

0.5 m H2SO4

1.28 V, 4 better

than IrO2

1.7 V, 141 better

than IrO2

≈80% after

200 cycles

[214]

S/N–C foam Exfoliated graphite via acid+thiourea,

autoclaved 12 h at 180 °C

N, S ≈ 1.18

and 0.51 at%

0.1 m KOH 1.55 V, 0.38 V, 98 77% after 16 h [215]

S/N–G/CNTs urea and thiourea treated GO+CNT,

autoclaved at 180 °C for 12 h

N-0.71, O-3.1,

S-1.26 at%

0.1 m KOH 1.57 V, 4 1.89 V, 103 – [216]

N/P-doped active

C-fiberCalcined and acid treated C paper+

electrochemical hydrogel growth in

aniline and ethylene diphosphonic

acid, pyrolysis 2 h at 700 °C

SA ≈ 473 m2 g−1 1 m KOH 1.51 V, 4 1.54 V, 87.4 93.4% after 12 h

(IrO2 ≈ 72.9%)

[217]

P/C3N4–C fiber Pyrolysis at 550 °C of hydro-

thermal treatment of c-paper in

melamine+ethylene diphosphonic

acidpyrolysis

N-13.2 wt%,

P-0.9 wt%

0.1 m KOH 1.53 V, 1.63 V, 61.6 93.4% after 30 h [218]

Material Synthesis method Chemical content and property

Electrolyte Onset potential [V] and n

Tafel slope [mV per decade]

Stability Ref.

P/C3N4 nanosheets

and –NH2-function-

alized carbon black

composite

Nitrilotris(methylene)-triphosphonic

acid with dicyandiamide-thermal

polycondensation at 600 °C—addition

of NH2-CB and freeze-drying

N-42.72, O-6.03

and P-4.53%,

SA ≈ 286 m2 g−1

0.1 m KOH 0.87 (Hg/HgO), 3.83 89 mV per

decade≈94% after

3000 cycles

[193]

N/F-codoped C

N/F–C aerogel Hydrothermal carbonization of glucose and

ammonium fluoride up to 1000 °C in N2

N-1.8, F-0.04 at%,

SA-768.4 m2 g−1

0.1 m KOH 0.912 (RHE), 3.7 70 mV per

decade

95% after 20 000 s [194]

Tridoped C

N/P/O–G Mixture of ball-milled Artemia cyst

shells and red P: KOH activation and

pyrolysis at 850 °C

O,N,P-9.12, 1.19 and

1.02 at% respect.

SA ≈ 1406 m2 g−1

0.1 m KOH 0.928 (RHE), 3.83 92% after 10000 s [89]

N/S/O hierarchical

C sheet with holes

Melamine, nickel sulfate and potassium

chloride ball milled and pyrolyzed at 800 °C

N-2.1, S-0.8 and O-3.8

at%, SA ≈ 576 m2 g−1

0.1 m KOH

0.5 m H2SO4

0.94 V (RHE), 3.85

≈0.9 V(RHE)≈95% after 100 h [30]

N/P/F graphene Pyrolysis (650 °C) of GO–PANi in

ammonium hexafluorophosphate (AHF)

N-7.11, P-0.37

and F-0.33 at%,

SA ≈ 512 m2 g−1

0.1 m KOH ≈0.93 (RHE), 3.85 100% after 1.4 h [57]

Table 3. Continued.



with a very low doping amount (0.5–1.26 at%), an efficient OER performance was also observed in S/N-codoped foam and rGO/CNT composite electrodes. This is known to be due to the syn-ergistic effects, modification in electronic configuration in the catalysts, and modulation in conducting properties.[215,216] How-ever though, the catalytic stability of the S/N-doped electrodes is lower as compared to that of the N- or N/O-doped carbon-based catalysts. As such, 3D carbon catalysts codoped with P/C3N4

[218] or P/N[217] showed an excellent OER stability (>93% current density retention) in alkaline medium (0.1–1 m KOH) for up to 30 h continuous operation. Here, the P and N codoping of the hierarchically porous carbon nanofibers were done by directly growing polymer nanowires on conductive carbon paper through electrochemical polymerization of aniline monomer in presence of phosphonic acid and followed by carbonization.[216] The obtained 3D carbon catalyst shown excellent stability (hardly any attenuation in continuous 12 h operation) and high OER performance with a relatively low (0.31 V at 10 mA cm−2) overpotential, which is comparatively close to that of iridium oxide (IrO2) and the precious benchmark.[217]

Experimental investigation has revealed that codoping carbon with P and N led to an increase in active surface area and the density of active sites in comparison to the single doped or pristine carbon-based counterparts. DFT calculations indicate that N and P dopants synergistically coactivate the adjacent C atoms, inducing enhanced activity toward OER.[217] This implies that N,P codoping can sufficiently decrease the total free energy for carbon framework as compared to that of pristine or even

single-doped carbon structures. If the potential is reduced to 0 V, the initial three steps, related to the electron transfer on adsorbing HO*, O*, and OOH*, respectively, become endothermic. How-ever, the fourth step remains exothermic which is related to desorption of O2. Interestingly, the third step relates to highest reaction energy barrier ultimately controlling the reaction rate-determining step. When the potential is raised close to 0.907 V (i.e., 0.505 V overpotential), all the above-mentioned elementary reaction pathways show downhill and exothermic characteristics triggering the OER to occur spontaneously. Figure 6 illustrates the initial structure of N,P-codoped carbon lattice (Figure 6a) and related structural modulation after adsorption of OH* (Figure 6b), O* (Figure 6c), and OOH* (Figure 6d) intermediates. The Gibb’s free energy diagrams of the OER catalytic reaction pathway on N,P-codoped carbon possessing different potentials infer sponta-neous reaction mechanism. Obviously, the electrocatalytic OER overpotential was considerably reduced through heteroatoms codoping. This observation prescribes that OER performance can be well improved by N,P-codoping in carbon structures.[217] In fact, the heteroatoms codoping into carbon materials have pro-duced numerous OER electrocatalysts with even multifunctional activities.[62,90,91,169,218–226]

Therefore, SN- and NP-codoped 3D carbon-based electrodes are most efficient for OER electrocatalysis. Particularly, heter-oatom (P and S)-doped C3N4-based porous carbon structures are excellent electrode candidate in this context. Codoping induced enhancement in OER efficiency by 3D carbon cata-lysts are often attributed to heteroatom induced charge

Adv. Mater. 2019, 1805598

Figure 6. Theoretical OER study: atomic structural details of: a) N- and P-codoped carbon and the corresponding modification after adsorption of: b) OH*, c) O*, and d) OOH* intermediates. e) Estimated free energy diagrams for the OER pathways on N/P-codoped carbon lattice at 0 V, 0.402 V (the equilibrium potential), and 0.907 V. f) Volcano plots of OER overpotential for differently doped metal-free and metal-based electrodes versus the of O* and OH* adsorption energy difference. g) The electronic density of states (DOS) for the differently doped carbon materials. a–g) Reproduced with permission.[217] Copyright 2017, Wiley-VCH.



redistribution and synergistic effects related to doping. There are various parameters related to 3D carbon materials to influ-ence onto OER activities which must be emphasized through more in-depth research studies.

7. 3D Porous Carbon-Based Electrocatalysts for Hydrogen Evolution Reaction HER

Although hydrogen’s superb emission-free fuel characteristics, hydrogen is mostly generated through steam reforming of nat-ural gas—a process that generates s carbon dioxide emissions (also observed in water gas shift reaction).[227,228] In contrast, electrochemical process of water reduction can produce H2 via hydrogen evolution reaction with no environment-degrading emission. However, the high cost of Pt, active electrocatalyst used for most HER, has impeded the large-scale commercial production of H2 through HER. Hence, it would be very impor-tant to develop cost-effective and efficient HER electrocata-lysts alternative to Pt. I this context, metal-free electrocatalysts derived from carbon have emerged to be an excellent alternative to Pt since several years. Henceforth, intense research efforts have been devoted in fabricating 2D carbon-based HER electro-catalysts.[229–232] The utilization of 3D carbon architectures for HER is a recent addition to this field.[233–240] Nevertheless, a few specific heteroatom-doped carbon have recently been demon-strated to exhibit attractive catalytic activity and stability toward HER in neutral, basic, as well as acidic conditions.

There are different mechanisms, Volmer/Herovsky/Tafel, for HER as shown below although these holds for other electro-chemical reactions, such as ORR and OER as well:[231]

H O e H HO ; Volmer 120 mV per decade2* ( )+ → +− −

H O e H H HO ; Heyrovsky 40 mV per decade2*

2 ( )+ + → +− −

H H H ; Tafel 30 mV per decade* *2 nn( )+ →

The Sabatier principle, most sensitive factor that influences HER rate, is the electrochemical balance between adsorption and desorption, which is, generally presented by a “volcano plot.”[230–232] Competing reactions determines HER rate, when the substrate interaction is too slow the Volmer reaction is inhibited, whereas, the Tafel/Heyrovksy reaction is suppressed when it is very strong. Thus, in regulating the HER process it is important in properly selecting interfacial properties between the catalyst and electrolyte layer along with ensuring a well-maintained sorption on the catalyst surface. In this regard, the relative adsorption free energy of the H* intermediate is used to evaluate HER performance for a catalyst and used as an indicator of the catalytic activity. Related to this, Conway and Tilak[230] developed a model for inspecting the underpoten-tial deposition coverage of hydrogen on reaction surface and derived the Tafel slopes associate with various mechanisms. The structural defects and heteroatom induced defects in carbon structures modulate the initial charge distribution to introduce electron accumulation area as the catalytic active site for HER.[2] This mechanistic understanding of the carbon-defect enhanced HER is applicable to all graphitic and active carbon structures.

Table 5 summarizes heteroatom-doped 3D carbon-based catalysts for HER.[234–240] Among them, the N-doped plasma

Adv. Mater. 2019, 1805598

Table 5. Heteroatom-doped 3D carbon-based catalysts for hydrogen evolution reaction.


Electrolyte onset potential [V vs RHE]

V at 10 mA cm−2 and Tafel slope

[mV per decade]

Stability Ref.

Microporous graphitic

N–C framework

1,2,4,5-Benzenetetramine

tetrahydrochloride condensation

reaction with octaketo-tetraphenylene

assisted with ZnCl2

SA ≈ 928 m2 g−1 0.5 m H2SO4 −0.26 −0.44, 121 – [234]

Poly(3,4-dinitro-

thiophene)/SWCNT

composite

Yamamoto polymerization:

SWNT+PDNT (41.9%) mixture

in DMF, pyrolyzed at 70 °C, and

acid wash

1 m H2SO4 −0.04 −0.12, - [235]

N-doped, plasma-

etched 3D graphene

Hydrothermal treatment of

GO+dopamine, 800 °C pyrolysis,

20–30 min Ar plasma etching

N-6.9 at% 0.5 m H2SO4

1 m KOH−0.06

−0.14

−0.13, 66

−0.22, 108

both ≈100%

after 20 h

[236]

gC3N4 nanoribbon

network

Autoclave of g-C3N4 and GO

suspended mixture at 180 °C≈23.74 at% 0.5 m H2SO4 −0.11 −0.27, 54 ≈130% after 15 h [237]

Porous

C3N4@N–grapheneCynamide+SiO2 spheres—

hydrothermal treatment, plus

GO+NH4OH, hydrazine reduction

N-4.6 at%,

SA-58 m2 g−1

0.5 m H2SO4 −0.008 −0.08, 49.1 86.3% after

5000 cycles

[238]

N/P–C nanofiber

network

Electropolymerization in phytic acid of

aniline (precursor for N and C), then

pyrolysis

Electrochemical

SA ≈ 325 mF cm−2

0.5 m H2SO4 −0.08 −0.15, 69 ≈100% after

2000 cycles

[239]

P graphene–C3N4

hybridPyrolysis of GO+triphenylphosphine,

900 °C, + dicyandiamide

lyophilization, pyrolysis at 600 °C

N-42.12, P-2.12

at%, SA-119 m2 g−1

0.5 m H2SO4

0.5 m KOH−0.076

−0.62

−0.34, 90- 97.53% after 8.3 h [240]



etched 3D graphene has shown promising HER performance in acid (0.5 m H2SO4) electrolytes with −0.06 V (RHE) onset potential.[236] For example, C3N4 networks of pristine or het-eroatom doping, have been tested for HER in both acid and alkaline media,[237,238,240] and the graphitic C3N4 nanoribbon network in 0.5 m H2SO4 electrolyte showed HER onset potential of −0.11 V (RHE) while P-doped graphene and C3N4 composite catalyst exhibited an onset potential of about −0.076 V (RHE).[237,240] Also worth to note that porous C3N4 nanolayers composited with N-doped graphene nanosheets revealed the lowest onset potential of −0.008 V (vs 0 mV for Pt/C, vs RHE @ 0.5 mA cm−2) in 0.5 m H2SO4 electrolyte with very good durability (negligible activity loss >5000 cycles).[238] This is due to the synergistic effects originated from i) 3D conductive gra-phene network, ii) hierarchical porous structure composition in the hybrid material, and iii) exfoliation of C3N4 into nanosheets and highly dense active sites produced by incorporation of pores on the plane of C3N4 sheets.[238] Also, other 3D carbon HER catalysts include poly(3,4-dinitrothiophene)/SWCNT com-posite[231] and N/P-codoped carbon nanofiber network.[239]

Heteroatom-doped or codoped 3D carbon materials are promising for efficient HER activities due to the charge transfer induced potential modulation on the electrode surface and high active surface area which facilitate the charge transport as well as mass transfer through the porous 3D network. However, the 3D nanostructure must be highly conductive for superior HER performance. In fact, 3D carbon nanostructures for HER appli-cation needs more research attention. Among doped carbon 3D nanomaterials C3N4-based electrodes doped with P or compos-ited with carbon allotropes have been used for HER electroca-talysis toward efficient water splitting. More research is needed in this respect to replace or even be compared with metal-based HER electrocatalytic performance.

8. 3D Carbon-Based Multifunctional Electrocatalysts

Multifunctional electrocatalysts, its inherent flexibility of enabling various redox reactions with significant cost reduc-tion, have attracted focused research attention. In this matter, bifunctional 3D carbon catalysts related to ORR and OER activities is summarized in Table 6. As can be inferred from data in Table 6, chemically and electrochemically synthesized (N, P, C3N4, and B)-doped and (N-C3N4, P/C3N4, P/S, B/N, and N/S)-codoped carbon 3D electrodes exhibited electrocatalytic activities for both ORR and OER.[62,90,91,169,218–226] As such, in bifunctional catalysts development, hydrothermal synthesis of N-doped mesoporous carbon nanosheet and MWCNT hybrid composite by pyrolyzing glucose, urea, and CNTs has shown to be an efficient approach.[220] The resulting hybrid exhib-ited notable OER performance showing low onset potential of about 1.50 V versus RHE and an overpotential of only 0.32 V at 10 mA cm−2 while an onset potential of 0.95 V foe ORR with an electron transfer number of 4 in alkaline medium (0.1 m KOH). The observed outstanding bifunctional catalytic activity is attributable to several factors, namely, mesoporous structure for a full exposure of active sites available to high specific sur-face area (594.1 m2 g−1), high N content (10.7 at%), improved

mass/electron transport, simplistic adsorption/release of oxygen bubbles, and a stable structure.[220]

Data in Table 6 reveal that heteroatom-doped C3N4 can exhibit excellent catalytic performance for ORR and OER both, though primarily in alkaline media (KOH). Particu-larly, P-doped C3N4 nanoflowers produced onto a carbon fiber paper showed onset potential of −0.94 V for ORR and 1.53 V for OER.[218] Furthermore, C nanocrystals (P,S-CNS; Figure 7) sandwiched in P/S-codoped C3N4 sponge demonstrated excep-tionally high specific surface area (1474 m2 g−1) and higher ORR and OER bifunctional catalytic activities than of Pt/C for ORR and RuO2 for OER, respectively, in spite of limiting cur-rent density and onset potential (Figure 8a–f).[91] The resultant electrode exhibited an outstanding durability and suitability as oxygen cathode in rechargeable primary Zn–air batteries. These Zn–air batteries indicated remarkable open-circuit voltage of 1.51 V, excellent discharge peak power density of 198 mW cm−2, notably high specific capacity of 830 mAh g−1, and robust durability even after 210 h mechanical recharging. An excel-lent reversibility and durability with favorable small polariza-tion in charge–discharge voltage (0.80 V at 25 mA cm−2) have also been reported for the Zn–air battery in three-electrode configuration (Figure 8g–j). DFT investigation reveals that the mechanism behind the exceptional electrocatalytic per-formance of P,S-CNS is due to the codoping-induced efficient mass/charge transport associated with high conductivity of the C3N4/C composite.[91]

It is obvious that recent rapid development of flexible and wearable electronics requires flexible power sources. However, processing and fabrication of flexible electrode fabrication for ORR and OER operations is still a big challenge.[90,91,169,218,223–242] In this context, the flexible and suf-ficiently large-area 3D mesoporous N-functionalized carbon microtube (NCMT) sponge is very interesting.[90] The struc-ture possessed micrometer-scale hollow core with graphitic interconnected pore walls. The NCMT sponge demonstrated very good electrocatalytic ORR and OER performance showing very small potential difference (0.63 V) in between OER (10 mA cm−2) and ORR (3 mA cm−2) current densities.[90] In another effort, a high-temperature pyrolysis method was used by Liu et al. to fabricate nanoporous carbon nanofiber films (abbreviated as, NCNFs). This porous film demonstrated excel-lent bifunctional catalytic performance for ORR and OER. The corresponding onset potential for ORR was 0.97 V versus RHE with a limiting current Density of 4.7 mA cm−2 and the OER onset potential was about 1.43 V with a value of 1.84 V at 10 mA cm−2. These values are very good as per the metal-free carbon-based multifunctional electrocatalysts. Furthermore, the NCNFs were flexible and have 1249 m2 g−1 specific surface area, 147 S m−1 electrical conductivity, 1.89 MPa tensile strength, and 0.31 GPa tensile modulus. These features made the elec-trode very attractive together with its excellence as air cathode for flexible, high-performance primary liquid ZABs (with high maximum power density of 185 mW cm−2 and specific capacity of 626 mAh g−1, and energy density of 776 Wh kg−1) and also for rechargeable liquid ZABs (tiny charge–discharge voltage gap of ≈0.73 V at 10 mA cm−2, high retentivity with an initial round-trip efficiency of 62%, excellent stability (voltage gap increased by ≈0.13 V after 500 cycles), and remarkable energy density

Adv. Mater. 2019, 1805598



(378 Wh kg−1).[242] No doubt, these flexible Zn–air batteries could be interesting energy supplier while integrated as flexible and wearable systems or devices for various applications.

Along with the advancement of 3D carbon-based as bifunc-tional catalysts for ORR–OER,[241,242] 3D carbon electrocatalysts for ORR–HER or OER–HER operations have recently been under intensive research,[243–246] as seen in Table 7. Among the 3D porous carbon bifunctional catalysts for HER and OER, carbon fiber–CNT–C3N4 composite and N–P–O-tridoped porous carbon are particularly promising.[244,246] Figure 9 shows the synthesis scheme and electrochemical evaluation for isotropic carbon-based heteroatom-doped metal-free electro-lyzer made of C3N4–CNT–CF and S–C3N4–CNT–CF as bifunc-tional catalysts for OER and HER in basic as well as acidic media.[241] The rationally designed architecture along with the 3D conducting CNT–CF hierarchy helped to show excellent

charge transport and also facilitated the electrolytic mass trans-port through the electrode. Also, the high quality dispersion and durability of the layered C3N4 structure enabled the excel-lent OER electrocatalytic activities (1.52 V onset potential vs RHE in 1 m KOH), and the sulfur doping improved the HER performance of the C3N4-based electrode with −0.15 V onset potential versus RHE in 0.5 m H2SO4 electrolyte.[244] The above attributes thus promote self-supported systems for proficient, cheap and environmental protected water splitting.[244] Further-more, the N–P–O-tridoped carbon-based catalyst performed HER and OER activities both in acid (0.5 m H2SO4) and alka-line (1 m KOH) medium (Table 7).[246] It is worth noting that, compared to bifunctional electrocatalysts, the development of tri/multifunctional carbon catalysts is still in infancy.[30,57] 3D heteroatom-doped carbon materials for CO2

[243–248,250–252] and H2O2

[253–255] reduction are also in preliminary stage and needs

Adv. Mater. 2019, 1805598

Table 6. Heteroatom-doped 3D porous carbon-based catalysts for bifunctional activities or ORR and OER.


Electrolyte onset potential [V vs RHE] and n

V at 10 mA cm−2 [V] and Tafel slope

[mV per decade]

Stability Ref.

O–C fiber (surface

exfoliated)

Edge/defect-rich and O-doped

graphene in situ exfoliation on

carbon fibers

O-13.9% 0.1 m KOH ORR 0.76 V, 3.5;

OER 1.58 V

OER 1.68 V, - Stable over 40 000 s [220]

N-CNT/G composite CVD-NCNT growth on rGO,

aided with ethylene diamine

N-3.9%,

SA-175.3 m2 g−1

0.1 m KOH ORR 0.97 V, 4;

OER 1.52 V, -

OER 1.67 V, - – [221]

N-mesoporous

C-nanosheet/CNT

hybrid (N-MCN/CNTs)

Urea and glucose hydrothermal

treatment of with MWCNTs,

800 °C pyrolysis in Ar

N-10.7, O-4.2 at%,

SA-594.1 m2 g−1

0.1 m KOH ORR 0.95 V, 4;

OER 1.5 V

ORR -, 79

OER 1.55 V, 55

ORR-92.5% (7 h),

OER-100% (14 h)

[222]

N–C microtube

sponge (flexible)

Pyrolysis of facial 100% cotton

at 1000 °C under NH3 for 1 hSA ≈ 2358 m2 g−1,

C, N, O—93.2,

2 and 4.8 at%

0.1 m KOH ORR 0.89 V, 4

OER 1.5 V

OER-1.52 V, 246 72.9% after 100 h

(Pt/C ≈ 50%)

[90]

N-doped carbon

nanonets,

g-N-MM-Cnet

TTIP/P123/HCl/H2O/ethanol

in mixture plus dicyandiamideSA ≈ 1947 m2 g−1 0.1 m KOH

0.5 m H2SO4

ORR 0.96 V, 3.9

OER 0.37 V

–, 70

1.6 V, 26

[223]

N–C/C3N4 Chitin +NaOH+ urea+ g-C3N4-

stirring, drying, pyrolysis

at 800 °C in Ar

N-13.19 at%,

SA ≈ 87.19 m2 g−1

0.1 m KOH ORR 0.86 V, 3.9

OER 1.64 V, –

–, 76

1.68 V, –

89.9%

86.8% after 50 000 s

[169]

P/C3N4 nitride

nanoflowers-on flexible

carbon-fiber paper

P/gC3N4 directly grown on CFP

(PCN-CFP) via 550 °C pyrolysis

0.1 m KOH ORR 0.94 V, -

OER 1.53 V

–, 122.3 [218]

P/S C3N4 sponge

sandwich with C

nanocrystals

Pyrolysis at 500 °C,

freeze-drying

N-41.36, P-1.68

and S-1.59%,

SA-1474 m2 g−1

0.1 m KOH ORR 0.97 V/4

OER 1.26 V

–, 61 (Pt/C 78)

1.56, 64

97.4% after 8000 s [91]

Heteroatom-doped

graphene “Idli”Microwave heating and 300 °C

calcination of heteroatom-

doped GO and rice flour

SA ≈ 499 m2 g−1 0.1 m KOH ORR −0.02 V, –

OER 0.29 V

–, 49.83

–, 35.71

[224]

B/N-porous C Calcination of silicon

sphere+methyl violet+boric

acid, then 800 °C and pyrolysis

N-7.94, O-9.48

and B-1.51 at%

0.1 m KOH ORR 0.92 V, 3.83

OER 1.19 V

1.23 V [225,249]

N/S porous C Teflon-assisted etching of silica

template and simultaneous

1100 °C pyrolysis of sucrose

and trithiocyanuric acid

N+S-3.21 and

O-3.15 wt%,

SA ≈ 830 m2 g−1

0.1 m KOH

0.1 m HClO4

0.1 m KOH

ORR 0.94 V,

3.86–3.96

0.88 V, 3.89–3.96

OER 1.66 V

1.69 V [226]

N/P mesoporous C Pyrolysis (1000 °C) of polyani-

line aerogel synthesized in the

presence of phytic acid

N ≈ 3.2, P ≈ 1.1,

O ≈ 4.9 at%,

SA ≈ 1584 m2 g−1

0.1 m KOH

0.1 m HClO4

0.1 m KOH

ORR 0.94 V, 3.85

ORR 0.64 V, 3.8

OER ≈1.3 V

–, 104

1.75 V, –

[241]



considerable attention. Thus, continued research in these emerging areas would be of great value.

3D carbon-based heteroatom-doped have long been applied for bifunctional electrocatalysis in ORR and OER together par-ticularly for metal–air batteries. It is crucial to fabricate innova-tive electrodes which perform efficiently for both ORR and OER to make commercial impact in this field. Heteroatom-codoped porous carbon materials have demonstrated proficient ORR or OER activity individually and are comparable to metal-based catalysts. However, their bifunctional (ORR and OER) property is often not conclusive. Trifunctional carbon-based efficient cat-alysts are even more rare. There are few reports on C3N4-based hybrid electrodes and NS- or NP- or NPO-doped 3D carbon materials with efficient ORR, OER, and HER performances.

9. 3D Carbon-Material-Based Multifunctional Integrated Energy Devices

Integrated energy systems for simultaneous energy conver-sion and storage applications have recently received immense technological and commercial interest. Fundamentally, a com-plete energy system should contain the energy conversion (e.g., solar cells, nanogen erator, among others), energy storage (such as Li-ion batteries, water splitting electrolyzers, super-capacitors, along with others innovative devices), and energy consuming units (e.g., photodetectors, pressure sensors, and magnetic sensors).[256–262] Due to technical difficulties in real-izing the complex integration and challenges of achieving the low-cost, highly efficient multifunctional catalysts, only a few

Adv. Mater. 2019, 1805598

Figure 7. a1) Schematic preparation of P,S-CNS catalysts, a2) reaction steps for production of the C−N polymeric complex. b) Digital images of P,S-CNS structure sponge. c,d) SEM, e) TEM, and f) HRTEM images of P,S-CNS nanostructure (inset: zoomed-in view). g) Related FFT pattern for the crystallite structure as shown in the inset of (f). h) TEM and elemental mapping of C, N, P, and S of P,S-CNS (scale, 300 nm). a–h) Reproduced with permission.[91] Copyright 2017, American Chemical Society.



Adv. Mater. 2019, 1805598

Figure 8. a) LSV plots for ORR in O2-saturated 0.1 m KOH medium (1600 rpm). b) LSVs of P,S-CNS of ORR at different rotational speeds; inset: K–L plots at different voltages. c) The HO2 yield produced in ORR and estimated electron transfer number of P,S-CNS. d) LSVs for OER in 0.1 m KOH electrolyte (1600 rpm). e) Tafel plots of ORR and OER. f) LSV curves for ORR and OER in 0.1 m KOH at 1600 rpm and 5 mV s−1 scan rate, g) charge–discharge cycles for rechargeable ZABs 2 mA cm−2 in two electrode configuration with P,S-CNS as air electrocatalyst, h) charge–discharge polarization plots of ZABs in three electrode configuration using commercial Pt/C, P-CNS, S-CNS, and P,S-CNS as air catalysts, i) schematic diagram of trielec-trode ZABs, and j) galvanostatic charge–discharge cycles for rechargeable ZABs in three electrode mode using P,S-CNS as bifunctional electrocatalyst. a–j) Reproduced with permission.[91] Copyright 2017, American Chemical Society.



Adv. Mater. 2019, 1805598

Figure 9. a) Schematic preparation process of self-supported C3N4-based metal-free electrolyzer, b) digital photograph, c,d) SEM images and e,f) HRTEM images of the C3N4–CNT–CF electrode, g) LSV and h) Tafel plots of CNT–CF, C3N4–CF, C3N4–CNT–CF, S–C3N4–CNT–CF and Pt in 0.5 m H2SO4 at 5 mV s−1. i) Chronopotentiometric response of S–C3N4–CNT–CF in 0.5 m H2SO4. j) LSV curve of S–C3N4–CNT–CF in 1 m KOH at 5 mV s−1 (inset shows the Tafel slope). a–j) Reproduced with permission.[160] Copyright 2016, The Royal Society of Chemistry. k) Preparation steps of melamine–phytic acid supermolecular aggregate, MPSA)/GO-1000 through cooperative assemble along with pyrolysis. Reproduced with permission.[55] Copyright 2016, Wiley-VCH.

Table 7. Heteroatom-doped metal-free carbon-based 3D catalysts for trifunctional activities toward ORR, OER, and HER.


Electrolyte onset potential [V vs RHE]

V at 10 mA cm−2 and Tafel slope

[mV per decade]

Stability Ref.

N-activated C Pyrolysis of activated C in NH3

flow at 500 °C for 3 h followed by

calcination at 1050 °C for 2 h

N-3.73 at% 0.5 m H2SO4

0.1 m KOHHER −0.34

ORR 0.882

0.34, 66

–, 57≈100% and 90.6%

after 20 000 s

[243]

S–C3N4/CNT/C fiber (HER)

C3N4/CNT/C fiber (OER)

Melamine solution soaking of clean

cotton cloth dipped into MWCNT

suspension; heating (90 °C),

calcined in Ar (550 °C), then heating

with S powder (450 °C)

SA-53.7 m2 g−1 0.5 m H2SO4

1 m KOHHER −0.15

OER 1.52

0.236, 81.6

1.6, 45

88.3$ after 12 h

89.6% after 14 h

[244]

N/P C network 1000 °C pyrolysis of melamine-phytic

acid supermolecular aggregate+GO

N ≈ 3 and

P ≈ 2.2 at%,

SA-375 m2 g−1

0.5 m H2SO4

0.1 m KOHHER −0.14

ORR 0.92

−0.16, 89

3.5 e

≈96% after 4 h- [55]

N/P/O-porous C Acid-oxidized carbon

cloth+ailine+phytic acid+ammonium

persulfate hydrothermal treatment;

1000 °C pyrolysis for 2 h in N2

N-0.46, P-0.32 and

O-16.4 at%

1 m KOH

1 m KOH

0.5 m H2SO4

HER −0.4

OER 1.64

HER −0.33

OER 1.7

−0.45, 154

1.65, 84

−0.39, 109

1.7, 200

≈95%, 5 h

≈97%, 5 h

≈80%, 5 h

≈90%, 5 h

[246]



Adv. Mater. 2019, 1805598

simple integrated energy storage devices have been realized for simultaneous energy conversion and energy storage, and/or as a power source for other electronic devices, such as photode-tectors and sensors.[261–271] Heteroatom-doped porous carbon materials have recently emerged as building blocks for efficient integrated energy systems and replace the expensive and scarce precious metal and other metal-oxide-based electrodes.[265,266]

A wire-shaped integrated energy conversion and storage device was fabricated after twisting two fiber-based elec-trodes.[270] One of the electrodes was composed of titanium oxide nanotube modified titanium wire and the other electrode was made of a fiber fabricated with multiwalled carbon nano-tubes and the electrolyte was prepared by incorporating an I3

−/I− redox ion couple into a redox element.[270] This wire-shaped

device performed as an efficient dye-sensitized solar cell with 6.58% energy conversion efficiency of 6.58% along with 85.03 µF cm−1 or 2.13 mF cm−2 specific capacitance. Further-more, the two functionalities could be altered successively without sacrificing performance.[270]

In another report, all solid-state and efficient self-powered “energy fiber” was fabricated, which could simultaneously con-vert solar energy to electric power and store it.[271] This opens up a new paradigm for advancement in photoelectronics and electronics technologies. MWCNT sheet was used for wrapping up a modified Ti wire to fabricate the PC panel, as illustrated in Figure 10a–e.[271] For the ES panel, poly(vinyl alcohol) (PVA)/H3PO4 gel electrolyte was applied onto the titania nanotube-modified Ti wire, and MWCNT sheets were mounted to make

Figure 10. Integrated polymer solar cell and electrochemical supercapacitor characteristics in flexible and stable fiber form: a) schematics of the circuit diagram b) charge–discharge characteristics at 0.1 µA current during discharge step, c) charge–discharge plots for the self-charged fiber at 0.1, 0.5, 1 to 5 µA current densities, d) cyclic voltammograms at 100–5000 mV s−1 scan rates, e) schematic representation of integrated, all-solid-state, coaxial fiber device(left and right panels are PC and ES, respectively). a–e) Reproduced with permission.[271] Copyright 2013, Wiley-VCH. Integrated perovskite cells with water splitting electrolyzer and a fuel cell: f) Schematic demonstration of the multifunctional hybrid system, g) current density versus voltage (J–V) characteristics of the perovskite tandem cell for dark and 100 mW cm−2 light exposure. h) J–t characteristics showing the stability and retentivity of the energy conversion performance of integrated water splitting device with on external bias under 100 mW cm−2 illumination. i) Gravimetric polarization curve and power density profile with current density of the integrated fuel cell using H2 and O2 collected from water splitting electrolyzer operated by the perovskite tandem cells. f–i) Reproduced with permission.[269] Copyright 2017, Elsevier Inc.



Adv. Mater. 2019, 1805598

the ES panel (Figure 10f–i).[271] The energy density obtained was 1.61 × 10−7 Wh cm−2 and the photoconversion and storage effi-ciency is found to depend on the thickness of MWCNT sheet in ES panel with a maximum value of 0.82% for 20 µm thick MWCNT layer and remained unchanged with further increase in thickness.[265] Furthermore, aqueous solar battery was also reported for directly converting solar energy and store as elec-trochemical energy in carbon nitride structures.[266]

Recently, N,S-codoped 3D graphitic framework (N,S-3DPG) was utilized to fabricate cost-effective, high efficient, trifunc-tional electrocatalyst for HER, OER, and ORR altogether. This technique combines simultaneous HER to produce H2 fuel through photo-electrochemical splitting of water, OER for O2 generation from water, and ORR toward clean electricity pro-duction in a fuel cell using the H2 and O2 gases obtained from HER and OER processes, respectively.[269] This multifunctional catalyst, performed light assisted electrochemical water splitting which was operated by CH3NH3PbI3 perovskite solar cells and generated clean energy in the form of electricity utilizing the obtained O2 and H2 gases in a H2–O2 fuel cell. Figure 10j shows the detailed mechanism of solar energy conversion and storage while Figure 10k–m presents the overall performance of the integrated energy system. The quantum efficiency of the solar cell itself was about 14.7%. The operational current density of the integrated device (Figure 10k) was 7.3 mA cm−2, which relates to 11.7% solar-to-hydrogen efficiency (Figure 10l).[269] Undoubtedly, such innovative efforts using metal-free carbon catalysts open new avenues for cheap, environmentally pro-tected and storage-free strategy for renewable production of clean energy, although more research is required for further optimization of critical parameters toward a commercial break-through for an energy sufficient future.[272]

10. Conclusions and Perspectives

To make a commercial impact in the field of electrocatalytic chemical energy storage and conversion, the costly metal and metal-oxide-based catalysts must be replaced by cost-effective, but highly efficient, alternatives, due to their low density, high specific surface area, excellent transport properties (both electrical and thermal), and suitable mechanical characteris-tics. 3D hierarchical mesoporous heteroatom-doped carbon nanostructures have evolved as promising metal-free cata-lysts to efficiently convert and store energy in various forms. 3D carbon-based hierarchical materials are generally synthe-sized by a chemical method or chemical vapor deposition techniques. The chemical method is cheaper and can prepare large quantity of electrode materials as compared to the CVD technique. However, 3D carbon materials fabricated by the chemical method are often inferior in structural homogeneity and intrinsic conducting properties. On the other hand, syn-thesizing actual 3D carbon structures with structural homoge-neity is still challenging. Therefore, more attention is required in preparing high-quality 3D carbon electrodes through CVD methods. In this regard, the 3D-printed technique seems very promising approach as an alternative. The activity of the heter-oatom-doped 3D carbon electrodes depends on their fabrication and activation techniques.

Furthermore, codoping or even tridoping of 3D mesoporous carbon materials led to multifunctional catalysts with similar or even better electrocatalytic performance than most of the precious metal and other transition-metal-oxide-based elec-trodes, opening up a new paradigm toward commercializa-tion of renewable energy technologies. However, the debates on the origin of electrocatalytic activity continues, especially regarding the recent advances in single-atom catalysts.[273–276] A trace amount of metal impurity can be present in carbon-based electrodes that are perceived as being metal-free catalyst. Researchers must be careful in this regard. Moreover, in-depth studies are undoubtedly required to enlighten the real mech-anism to achieve ideally excellent electrode with multifunc-tional (ORR, OER, and HER) activities. This insight can make advancement in the carbon-based metal-free electrocatalysis for various energy storage and conversion applications. Conclu-sively, it can be noted that more research efforts are required to utilize 3D heteroatom-doped mesoporous carbon electrodes for integrating various electrochemical energy conversion sys-tems with solar, thermal, mechanical, as well as other electro-chemical energy storage systems for efficient, self-powered, and energy sufficient future.

AcknowledgementsThis work was financially supported by the U.S. Air Force Office of Scientific Research (FA9550-12-1-0037), NASA (NNX16AD48A), The National Key Research and Development Program of China (2017YFA0206500), and The National Natural Science Foundation of China (51732002 and 21620102007). The authors would like to thank NSF, NASA, and the Department of Defense-Multidisciplinary University Research Initiative (MURI) for financial assistance. Many thanks are also due to our colleagues, collaborators, and peers for their work cited in this article.

Conflict of InterestThe authors declare no conflict of interest.

Keywords3D heteroatoms, energy devices, metal-free catalysts, nanoporous carbon

Received: August 28, 2018Revised: November 25, 2018

Published online:

[1] V. Georgakilas, J. A. Perman, J. Tucek, R. Zboril, Chem. Rev. 2015, 115, 4744.

[2] X. Liu, L. Dai, Nat. Rev. Mater. 2016, 1, 16064.[3] L. Dai, Y. Xue, L. Qu, H. J. Choi, J. B. Baek, Chem. Rev. 2015, 115,

4823.[4] Y. Chen, L. Dai, Y. Ohno, Carbon 2018, 126, 621.[5] Z. Xiang, Q. Dai, J. Chen, L. Dai, Adv. Mater. 2016, 28, 6253.[6] A. Erdemir, G. Ramirez, O. L. Eryilmaz, B. Narayanan, Y. Liao,

G. Kamath, S. K. R. S. Sankaranarayanan, Nature 2016, 536, 67.[7] R. Paul, Nano-Struct. Nano-Objects 2017, 10, 69.



Adv. Mater. 2019, 1805598

[8] I. Vlasov, O. I. Lebedev, V. G. Ralchenko, E. Goovaerts, G. Bertoni, G. V. Tendeloo, V. I. Konov, Adv. Mater. 2007, 19, 4058.

[9] J. Xiao, G. Ouyang, P. Liu, C. X. Wang, G. W. Yang, Nano Lett. 2014, 14, 3645.

[10] J. T. Kim, U. Choudhury, H. H. Jeong, P. Fischer, Adv. Mater. 2017, 29, 1701024.

[11] R. Paul, A. A. Voevodin, D. Zemlyanov, A. K. Roy, T. S. Fisher, Adv. Funct. Mater. 2012, 22, 3682.

[12] H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, R. E. Smalley, Nature 1985, 318, 162.

[13] W. Kratschmer, L. D. Lamb, K. Fostiropoulos, D. R. Huffman, Nature 1990, 347, 354.

[14] S. Iijima, Nature 1991, 354, 56.[15] S. Iijima, T. Ichihashi, Nature 1993, 363, 603.[16] X. Chen, R. Paul, L. Dai, Natl. Sci. Rev. 2017, 4, 453.[17] R. Paul, V. Etacheri, V. G. Pol, J. Hu, T. S. Fisher, RSC Adv. 2016,

6, 79734.[18] H. Tetsuka, A. Nagoya, T. Fukusumi, T. Matsui, Adv. Mater. 2016,

28, 4632.[19] X. Miao, D. Qu, D. Yang, B. Nie, Y. Zhao, H. Fan, Z. Sun,

Adv. Mater. 2018, 30, 1704740.[20] J. Mao, J. Iocozzia, J. Huang, K. Meng, Y. Lai, Z. Lin, Energy

Environ. Sci. 2018, 11, 772.[21] Y. Wang, Y. Pan, L. Zhu, N. Guo, R. Wang, Z. Zhang, S. Qiu, Inorg.

Chem. Front. 2018, 5, 656.[22] X. Gui, Z. Zeng, Y. Zhu, H. Li, Z. Lin, Q. Gan, R. Xiang, A. Cao,

Z. Tang, Adv. Mater. 2014, 26, 1248.[23] R. Paul, L. Dai, Compos. Interfaces 2018, 25, 539.[24] H. Wang, X. B. Li, L. Gao, H. L. Wu, J. Yang, L. Cai, T. B. Ma,

C. H. Tung, L. Z. Wu, G. Yu, Angew. Chem. 2018, 130, 198.[25] J. Han, D. Kong, W. Lv, D. M. Tang, D. Han, C. Zhang, D. Liu,

Z. Xiao, X. Zhang, J. Xiao, X. He, F. C. Hsia, C. Zhang, Y. Tao, D. Golberg, F. Kang, L. Zhi, Q. H. Yang, Nat. Commun. 2018, 9, 402.

[26] B. Qiu, M. Xing, J. Zhang, Chem. Soc. Rev. 2018, 47, 2165.[27] X. Li, L. Zhi, Chem. Soc. Rev. 2018, 47, 3189.[28] T. Lv, M. Liu, D. Zhu, L. Gan, T. Chen, Adv. Mater. 2018, 30,

1705489.[29] Y. Xue, Y. Ding, J. Niu, Z. Xia, A. Roy, H. Chen, J. Qu, Z. L. Wang,

L. Dai, Sci. Adv. 2015, 1, e1400198.[30] C. Hu, L. Dai, Adv. Mater. 2017, 29, 1604942.[31] Y. He, B. Matthews, J. Wang, L. Song, X. Wang, G. Wu, J. Mater.

Chem. A 2018, 6, 735.[32] J. Shi, Y. Dong, T. Fisher, X. Ruan, J. Appl. Phys. 2015, 118, 044302.[33] K. Shehzad, Y. Xu, C. Gao, X. Duan, Chem. Soc. Rev. 2016, 45,

5541.[34] Z. Y. Wu, H. W. Liang, L. F. Chen, B. C. Hu, S. H. Yu, Acc. Chem.

Res. 2016, 49, 96.[35] J. Zhang, F. Zhao, Z. Zhang, N. Chen, L. Qu, Nanoscale 2013, 5,

3112.[36] D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun,

A. Slesarev, L. B. Alemany, W. Lu, J. M. Tour, ACS Nano 2010, 4, 4806.[37] R. Paul, R. N. Gayen, S. Biswas, S. V. Bhat, R. Bhunia, RSC Adv.

2016, 6, 61661.[38] Y. Zhu, L. Peng, Z. Fang, C. Yan, X. Zhang, G. Yu, Adv. Mater. 2018,

30, 1706347.[39] W. Lv, Z. Li, Y. Deng, Q. H. Yang, F. Kang, Energy Storage Mater.

2016, 2, 107.[40] Y. Wang, Z. Wang, J. Mater. Res. 2018, 33, 1058.[41] M. Shao, Q. Chang, J. P. Dodelet, R. Chenitz, Chem. Rev. 2016,

116, 3594.[42] J. Wu, Y. Xue, X. Yan, W. Yan, Q. Cheng, Y. Xie, Nano Res. 2012, 5,

521.[43] T. Ohmori, H. Takahashi, H. Mametsuka, E. Suzuki, Phys. Chem.

Chem. Phys. 2000, 2, 3519.

[44] M. W. Kanan, D. G. Nocera, Science 2008, 321, 1072.[45] A. Goux, T. Pauporte, D. Lincot, Electrochim. Acta 2006, 51, 3168.[46] J. A. Seaboldand, K. S. Choi, Chem. Mater. 2011, 23, 1105.[47] X. Li, F. C. Walsh, D. Pletcher, Phys. Chem. Chem. Phys. 2011, 13,

1162.[48] L. Dai, Curr. Opin. Electrochem. 2017, 4, 18.[49] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Science 2009,

323, 760.[50] J. Shui, M. Wang, F. Du, L. Dai, Sci. Adv. 2015, 1, e1400129.[51] Y. Zhao, R. Nakamura, K. Kamiya, S. Nakanishi, K. Hashimoto,

Nat. Commun. 2013, 4, 2390.[52] J. T. Zhang, Z. H. Zhao, Z. H. Xia, L. Dai, Nat. Nanotechnol. 2015,

10, 444.[53] Y. Zheng, Y. Jiao, L. H. Li, T. Xing, Y. Chen, M. Jaroniec, S. Z. Qiao,

ACS Nano 2014, 8, 5290.[54] J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz,

S. T. Lee, J. Zhong, Z. Kang, Science 2015, 347, 970.[55] J. Zhang, L. Qu, G. Shi, J. Liu, J. Chen, L. Dai, Angew. Chem., Int.

Ed. 2016, 55, 2230.[56] B. Kumar, M. Asadi, D. Pisasale, S. S. Ray, B. A. Rosen, R. Haasch,

J. Abiade, A. L. Yarin, A. S. Khojin, Nat. Commun. 2013, 4, 2189.[57] J. Zhang, L. Dai, Angew. Chem., Int. Ed. 2016, 55, 13296.[58] B. Men, Y. Sun, M. Li, C. Hu, M. Zhang, L. Wang, Y. Tang, Y. Chen,

P. Wan, J. Pan, ACS Appl. Mater. Interfaces 2016, 8, 1415.[59] Z. Lin, G. H. Waller, Y. Liu, M. Liu, C. Wong, Nano Energy 2013, 2,

241.[60] R. L. Liu, D. Q. Wu, X. L. Feng, K. Mullen, Angew. Chem., Int. Ed.

2010, 49, 2565.[61] L. Chen, R. Du, J. Zhu, Y. Mao, C. Xue, N. Zhang, Y. Hou, J. Zhang,

T. Yi, Small 2015, 11, 1423.[62] H. Jin, H. Zhang, H. Zhong, J. Zhang, Energy Environ. Sci. 2011, 4,

3389.[63] Y. Ito, H. J. Qiu, T. Fujita, Y. Tanabe, K. Tanigaki, M. Chen, Adv.

Mater. 2014, 26, 4145.[64] W. Ding, L. Li, K. Xiong, Y. Wang, W. Li, Y. Nie, S. Chen, X. Qi,

Z. Wei, J. Am. Chem. Soc. 2015, 137, 5414.[65] Z. Wen, S. Ci, Y. Hou, J. Chen, Angew. Chem. 2014, 126, 6614.[66] H. B. Yang, J. Miao, S. F. Hung, J. Chen, H. B. Tao, X. Wang,

L. Zhang, R. Chen, J. Gao, H. M. Chen, L. Dai, B. Liu, Sci. Adv. 2016, 2, e1501122.

[67] Y. Zheng, Y. Jiao, J. Chen, J. Liu, J. Liang, A. Du, W. Zhang, Z. Zhu, S. C. Smith, M. Jaroniec, G. Q. Lu, S. Z. Qiao, J. Am. Chem. Soc. 2011, 133, 20116.

[68] M. Zhang, Q. Dai, H. Zheng, M. Chen, L. Dai, Adv. Mater. 2018, 30, 10.[69] L. L. Zhang, X. Zhao, H. Ji, M. D. Stoller, L. Lai, S. Murali,

S. Mcdonnell, B. Cleveger, R. M. Wallace, R. S. Ruoff, Energy Environ. Sci. 2012, 5, 9618.

[70] S. N. Faisal, E. Haque, N. Noorbehesht, W. Zhang, A. T. Harris, T. L. Church, A. I. Minett, RSC Adv. 2017, 7, 17950.

[71] S. Yang, X. Feng, X. Wang, K. Mullen, Angew. Chem., Int. Ed. 2011, 50, 5339.

[72] J. Wu, L. Ma, R. M. Yadav, Y. Yang, X. Zhang, R. Vajtai, J. Lou, P. M. Ajayan, ACS Appl. Mater. Interfaces 2015, 7, 14763.

[73] J. Tian, R. Ning, Q. Liu, A. M. Asiri, A. O. A. Youbi, X. Sun, ACS Appl. Mater. Interfaces 2014, 6, 1011.

[74] Y. Zheng, Y. Jiao, L. Hua, T. Xing, Y. Chen, M. Jaroniec, S. Z. Qiao, ACS Nano 2014, 8, 5290.

[75] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, Nat. Mater. 2015, 14, 271.

[76] S. A. M. Silva, J. Pereza, R. M. Torresia, C. A. Luengob, E. A. Ticianellia, Electrochim. Acta 1999, 44, 3565.

[77] L. Sun, C. Wang, Y. Zhou, X. Zhang, J. Qiu, J. Appl. Electrochem. 2014, 44, 309.

[78] W. Gajewski, P. Achatz, O. A. Williams, K. Haenen, E. Bustarret, M. Stutzmann, J. A. Garrido, Phys. Rev. B 2009, 79, 45206.



Adv. Mater. 2019, 1805598

[79] M. A. Angadi, T. Watanabe, A. Bodapati, X. C. Xiao, O. Auciello, J. A. Carlisle, J. A. Eastman, P. Keblinski, P. K. Schelling, S. R. Phillpot, J. Appl. Phys. 2006, 99, 114301.

[80] O. A. Williams, Diamond Relat. Mater. 2011, 20, 621.[81] Z. Han, A. Fin, Prog. Polym. Sci. 2011, 36, 914.[82] C. Lu, C. Liu, J. Chem. Technol. Biotechnol. 2006, 81, 1932.[83] W. Wan, Y. Lin, A. Prakash, Y. Zhou, J. Mater. Chem. A 2016, 4,

18687.[84] Y. Wang, H. Liu, K. Wang, S. Song, P. Tsiakaras, Appl. Catal., B

2017, 210, 57.[85] D. D. L. Chung, Appl. Therm. Eng. 2001, 21, 1593.[86] M. Fujii, X. Zhang, H. Xie, H. Ago, K. Takahashi, T. Ikuta, H. Abe,

T. Shimizu, Phys. Rev. Lett. 2005, 95, 065502.[87] G. Tao, L. Zhang, L. Chen, X. Cui, Z. Hua, M. Wang, J. Wang,

Y. Chen, J. Shi, Carbon 2015, 86, 108.[88] B. You, F. Kang, P. Yin, Q. Zhang, Carbon 2016, 103, 9.[89] Y. Zhao, S. Huang, M. Xia, S. Rehman, S. Mu, Z. Kou, Z. Zhang,

Z. Chen, F. Gao, Y. Hou, Nano Energy 2016, 28, 346.[90] J. C. Li, P. X. Hou, S. Y. Zhao, C. Liu, D. M. Tang, M. Cheng,

F. Zhang, H. M. Cheng, Energy Environ. Sci. 2016, 9, 3079.[91] S. S. Shinde, C. H. Lee, A. Sami, D. H. Kim, S. U. Lee, J. H. Lee,

ACS Nano 2017, 11, 347.[92] S. H. Lee, H. W. Kim, J. O. Hwang, W. J. Lee, J. Kwon,

C. W. Bielawski, R. S. Ruoff, S. O. Kim, Angew. Chem., Int. Ed. 2010, 49, 10084.

[93] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H. M. Cheng, Nat. Mater. 2011, 10, 424.

[94] C. Xu, B. Xu, Y. Gu, Z. Xiong, J. Sun, X. S. Zhao, Energy Environ. Sci. 2013, 6, 1388.

[95] B. G. Choi, M. Yang, W. H. Hong, J. W. Choi, Y. S. Huh, ACS Nano 2012, 6, 4020.

[96] L. Tian, M. J. Meziani, F. Lu, C. Y. Kong, L. Cao, T. J. Thorne, Y. Sun, ACS Appl. Mater. Interfaces 2010, 2, 3217.

[97] Y. Liu, Q. Feng, X. Xie, X. Ye, Carbon 2011, 49, 3371.[98] Y. Pan, H. Bao, L. Li, ACS Appl. Mater. Interfaces 2011, 3, 4819.[99] M. Kotal, A. K. Bhowmick, J. Phys. Chem. C 2013, 117, 25865.

[100] V. C. Tung, L. Chen, M. J. Allen, J. K. Wassei, K. Nelson, R. B. Kaner, Y. Yang, Nano Lett. 2009, 9, 1949.

[101] C. Zhang, S. Huang, W. W. Tjiu, W. Fan, T. X. Liu, J. Mater. Chem. 2012, 22, 2427.

[102] M. K. Shin, B. Lee, S. H. Kim, J. A. Lee, G. M. Spinks, S. Gambhir, G. G. Wallace, M. E. Kozlov, R. H. Baughman, S. J. Kim, Nat. Commun. 2012, 3, 650.

[103] I. Shakir, Electrochim. Acta 2014, 129, 396.[104] J. Wang, J. Tang, B. Ding, V. Malgras, Z. Chang, X. Hao, Y. Wang,

H. Dou, X. Zhang, Y. Yamauchi, Nat. Commun. 2017, 8, 15717.[105] S. C. Wang, J. Yang, X. Y. Zhou, J. Li, Int. J. Electrochem. Sci. 2013,

8, 9692.[106] T. Hong, D. W. Lee, H. J. Choi, H. S. Shin, B. Kim, ACS Nano 2010,

4, 3861.[107] E. Nagelli, L. Huang, A. Dai, F. Du, L. Dai, Part. Part. Syst. Charact.

2017, 34, 1700131.[108] D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. Dommett,

G. Evmenenko, S. T. Nguyen, R. S. Ruoff, Nature 2007, 448, 457.[109] J. Huang, Z. Xu, S. Abouali, M. A. Garakani, J. Kim, Carbon 2016,

99, 624.[110] X. Chen, M. Qiu, H. Ding, K. Fu, Y. Fan, Nanoscale 2016, 8, 5696.[111] Z. Yang, Y. Zhao, Q. Xiao, Y. Zhang, L. Jing, Y. Yan, K. Sun, ACS

Appl. Mater. Interfaces 2014, 6, 8497.[112] D. D. Nguyen, N. H. Tai, S. Y. Chen, Y. L. Chueh, Nanoscale 2012, 4, 632.[113] K. Kumar, Y. Kim, X. Li, J. Ding, F. T. Fisher, E. Yang, Chem. Mater.

2013, 25, 3874.[114] D. H. Lee, J. E. Kim, T. H. Han, J. W. Hwang, S. Jeon, S. Choi,

S. H. Hong, W. J. Lee, R. S. Ruoff, S. O. Kim, Adv. Mater. 2010, 22, 1247.

[115] Z. J. Fan, J. Yan, L. J. Zhi, Q. Zhang, T. Wei, J. Feng, M. L. Zhang, W. Z. Qian, F. Wei, Adv. Mater. 2010, 22, 3723.

[116] B. S. Mao, Z. Wen, Z. Bo, J. Chang, X. Huang, J. Chen, ACS Appl. Mater. Interfaces 2014, 6, 9881.

[117] X. C. Dong, Y. W. Ma, G. Y. Zhu, Y. X. Huang, J. Wang, M. B. Chan-Park, L. H. Wang, W. Huang, P. Chen, J. Mater. Chem. 2012, 22, 17044.

[118] W. Zhang, H. Xie, R. Zhang, M. Jian, C. Wang, Q. Zheng, F. Wei, Y. Zhang, Carbon 2015, 86, 358.

[119] J. Jiang, Y. Li, C. Gao, N. D. Kim, X. Fan, G. Wang, Z. Peng, R. H. Hauge, J. M. Tour, ACS Appl. Mater. Interfaces 2016, 8, 7356.

[120] Y. Zhao, C. Hu, L. Song, L. Wang, G. Shi, L. Dai, L. Qu, Energy Environ. Sci. 2014, 7, 1913.

[121] C. Tang, Q. Zhang, M. Zhao, J. Huang, X. Cheng, G. Tian, H. Peng, F. Wei, Adv. Mater. 2014, 26, 6100.

[122] M. Q. Zhao, X. F. Liu, Q. Zhang, G. L. Tian, J. Q. Huang, W. C. Zhu, F. Wei, ACS Nano 2012, 6, 10759.

[123] T. Chen, Q. Zhang, M. Zhao, J. Huang, C. Tang, F. Wei, Carbon 2015, 95, 292.

[124] H. Peng, J. Huang, M. Zhao, Q. Zhang, X. Cheng, X. Liu, W. Qian, F. Wei, Adv. Funct. Mater. 2014, 24, 2772.

[125] C. Tang, Q. Zhangn, M. Q. Zhao, G. L. Tian, F. Wei, Nano Energy 2014, 7, 161.

[126] F. Du, D. Yu, L. Dai, S. Ganguli, V. Varshney, A. K. Roy, Chem. Mater. 2011, 23, 4810.

[127] Y. Xue, D. Yu, L. Dai, R. Wang, D. Li, A. Roy, F. Lu, H. Chen, Y. Liu, J. Qu, Phys. Chem. Chem. Phys. 2013, 15, 12220.

[128] Z. Yang, M. Liu, C. Zhang, W. Tjiu, T. Liu, H. Peng, Angew. Chem., Int. Ed. 2013, 52, 3996.

[129] M. K. Liu, Y. E. Miao, C. Zhang, W. W. Tjiu, Z. B. Yang, H. S. Peng, T. X. Liu, Nanoscale 2015, 7, 1037.

[130] L. Borchardt, Q. L. Zhu, M. E. Casco, R. Berger, X. Zhuang, S. Kaskel, X. Feng, Q. Xu, Mater. Today 2017, 20, 592.

[131] S. Lawes, A. Riese, Q. Sun, N. Cheng, X. Sun, Carbon 2015, 92, 150.

[132] M. Wei, F. Zhang, W. Wang, P. Alexandridis, C. Zhou, G. Wu, J. Power Sources 2017, 354, 134.

[133] Y. Yang, X. Li, X. Zheng, Z. Chen, Q. Zhou, Y. Chen, Adv. Mater. 2018, 30, 1704912.

[134] S. D. Lacey, D. J. Kirsch, Y. Li, J. T. Morgenstern, B. C. Zarket, Y. Yao, J. Dai, L. Q. Garcia, B. Liu, T. Gao, S. Xu, S. R. Raghavan, J. W. Connell, Y. Lin, L. Hu, Adv. Mater. 2018, 30, 1705651.

[135] C. Zhu, T. Liu, F. Qian, T. Y. J. Han, E. B. Duoss, J. D. Kuntz, C. M. Spadaccini, M. A. Worsley, Y. Li, Nano Lett. 2016, 16, 3448.

[136] C. Zhu, T. Y. J. Han, E. B. Duoss, A. M. Golobic, J. D. Kuntz, C. M. Spadaccini, M. A. Worsley, Nat. Commun. 2015, 6, 6962.

[137] K. Chi, Z. Zhang, J. Xi, Y. Huang, F. Xiao, S. Wang, Y. Liu, ACS Appl. Mater. Interfaces 2014, 6, 16312.

[138] Y. Zhang, L. Guo, S. Wei, Y. He, H. Xia, Q. Chen, H. B. Sun, F. S. Xiao, Nano Today 2010, 5, 15.

[139] W. Gao, N. Singh, L. Song, Z. Liu, A. L. M. Reddy, L. Ci, R. Vajtai, Q. Zhang, B. Wei, P. M. Ajayan, Nat. Nanotechnol. 2011, 6, 496.

[140] J. Lin, Z. Peng, Y. Liu, F. R. Zepeda, R. Ye, E. L. G. Samuel, M. J. Yacaman, B. I. Yakobson, J. M. Tour, Nat. Commun. 2014, 5, 5714.

[141] R. Ye, D. James, J. M. Tour, Acc. Chem. Res. 2018, 51, 1609.[142] M. Andersson, S. B. Beale, M. Espinoza, Z. Wu, W. Lehnert, Appl.

Energy 2016, 180, 757.[143] E. Yeager, J. Mol. Catal. 1986, 38, 5.[144] A. J. Bard, L. R. Faulkner, Electrochemical Methods: Fundamentals

and Applications, Wiley, New York 2001.[145] L. Qu, Y. Liu, J. B. Baek, L. Dai, ACS Nano 2010, 4, 1321.[146] J. S. Lee, K. Jo, T. Lee, T. Yun, J. Cho, B. S. Kim, J. Mater. Chem. A

2013, 1, 9603.[147] Y. Yang, H. Chang, J. Mater. Res. 2018, 33, 1247.



Adv. Mater. 2019, 1805598

[148] H. Li, W. Kang, L. Wang, Q. Yue, S. Xu, H. Wang, J. Liu, Carbon 2013, 54, 249.

[149] X. Zhou, Z. Bai, M. Wu, J. Qiao, Z. Chen, J. Mater. Chem. A 2015, 3, 3343.

[150] S. A. Wohlgemuth, T. P. Fellinger, P. Jaker, M. Antonietti, J. Mater. Chem. A 2013, 1, 4002.

[151] Q. Guo, D. Zhao, S. Liu, S. Chen, M. Hanif, H. Hou, Electrochim. Acta 2014, 138, 318.

[152] J. Liu, B. V. Cunning, T. Daio, A. Mufundirwa, K. Sasaki, S. M. Lythf, Electrochim. Acta 2016, 220, 554.

[153] H. Jin, J. Li, F. Chen, L. Gao, H. Zhang, D. Liu, Q. Liu, Electrochim. Acta 2016, 222, 438.

[154] X. Zhou, S. Tang, Y. Yin, S. Sun, J. Qiao, Appl. Energy 2016, 175, 459.

[155] S. Tang, X. Zhou, N. Xu, Z. Bai, J. Qiao, J. Zhang, Appl. Energy 2016, 175, 405.

[156] W. Hu, N. Yoshida, Y. Hirota, S. Tanaka, N. Nishiyama, Electrochem. Commun. 2017, 75, 9.

[157] H. W. Liang, Z. Y. Wu, L. F. Chen, C. Li, S. H. Yun, Nano Energy 2015, 11, 366.

[158] L. T. Song, Z. Y. Wu, H. W. Liang, F. Zhou, Z. Y. Yu, L. Xu, Z. Pan, S. H. Yun, Nano Energy 2016, 19, 117.

[159] C. Hu, Y. Xiao, Y. Zhao, N. Chen, Z. Zhang, M. Cao, L. Qu, Nanoscale 2013, 5, 2726.

[160] D. Yu, L. Wei, W. Jiang, H. Wang, B. Sun, Q. Zhang, K. Goh, R. Si, Y. Chen, Nanoscale 2013, 5, 3457.

[161] H. W. Liang, X. Zhuang, S. Bruller, X. Feng, K. Mullen, Nat. Commun. 2014, 5, 4973.

[162] C. Xuan, Z. Wu, W. Lei, J. Wang, J. Guo, D. Wang, ChemCatChem 2017, 9, 809.

[163] M. Wu, K. Wang, M. Yi, Y. Tong, Y. Wang, S. Song, ACS Catal. 2017, 7, 6082.

[164] D. Eisenberg, W. Stroek, N. J. Geels, S. Tanase, M. Ferbinteanu, S. J. Teat, P. Mettraux, N. Yan, G. Rothenberg, Phys. Chem. Chem. Phys. 2016, 18, 20778.

[165] L. Li, P. Dai, X. Gu, Y. Wang, L. Yan, X. Zhao, J. Mater. Chem. A 2017, 5, 789.

[166] Y. Fu, C. Tian, F. Liu, L. Wang, H. Yan, B. Yang, Nano Res. 2016, 9, 3364.

[167] Z. Wen, S. Ci, Y. Hou, J. Chen, Angew. Chem. 2014, 126, 6614.[168] D. Yu, L. Zhou, J. Tang, J. Li, J. Hu, C. Peng, H. Liu, Ind. Eng.

Chem. Res. 2017, 56, 8880.[169] X. Wu, S. Li, B. Wang, J. Liu, M. Yu, Microporous Mesoporous Mater.

2017, 240, 216.[170] J. Liang, Y. Zheng, J. Chen, J. Liu, D. H. Jurcakova, M. Jaroniec,

S. Z. Qiao, Angew. Chem. 2012, 124, 3958.[171] X. Fu, X. Hu, Z. Yan, K. Lei, F. Li, F. Cheng, J. Chen, Chem.

Commun. 2016, 52, 1725.[172] M. Seredych, K. László, E. R. Castellón, T. J. Bandosz, J. Energy

Chem. 2016, 25, 236.[173] L. Chen, X. Cui, Y. Wang, M. Wang, R. Qiu, Z. Shu, L. Zhang,

Z. Hua, F. Cui, C. Wei, J. Shi, Dalton Trans. 2014, 43, 3420.[174] M. Seredych, K. Laszlo, T. J. Bandosz, ChemCatChem 2015, 7,

2924.[175] I. Y. Jeon, S. Zhang, L. Zhang, H.-J. Choi, J.-M. Seo, Z. Xia, L. Dai,

J.-B. Baek, Adv. Mater. 2013, 25, 6138.[176] Y. Su, Y. Zhang, X. Zhuang, S. Li, D. Wu, F. Zhang, X. Feng, Carbon

2013, 62, 296.[177] S. Jiang, Y. Sun, H. Dai, J. Hu, P. Ni, Y. Wang, Z. Lia, Electrochim.

Acta 2015, 174, 826.[178] H. Wu, L. Shi, J. Lei, D. Liu, D. Qu, Z. Xie, X. Du, P. Yang, X. Hu,

J. Li, H. Tang, J. Power Sources 2016, 323, 90.[179] S. Gao, H. Liu, K. Geng, X. Wei, Nano Energy 2015, 12, 785.[180] Z. Wu, R. Liu, J. Wang, J. Zhu, W. Xiao, C. Xuan, W. Lei, D. Wang,

Nanoscale 2016, 8, 19086.

[181] S. Wang, E. Iyyamperumal, A. Roy, Y. Xue, D. Yu, L. Dai, Angew. Chem., Int. Ed. 2011, 50, 11756.

[182] S. A. Wohlgemuth, R. J. White, M. G. Willinger, M. M. Titirici, M. Antonietti, Green Chem. 2012, 14, 1515.

[183] M. Wu, J. Qiao, K. Li, X. Zhou, Y. Liu, J. Zhang, Green Chem. 2016, 18, 2699.

[184] Z. Huang, H. Zhou, W. Yang, C. Fu, L. Chen, Y. Kuang, ChemCatChem 2017, 9, 987.

[185] M. Sahoo, S. Ramaprabhu, Energy 2017, 119, 1075.[186] S. Fu, C. Zhu, J. Song, M. H. Engelhard, X. Li, P. Zhang, H. Xia,

D. Du, Y. Lin, Nano Res. 2017, 10, 1888.[187] Z. Wang, X. Cao, J. Ping, Y. Wang, T. Lin, X. Huang, Q. Ma,

F. Wang, C. He, H. Zhang, Nanoscale 2015, 7, 9394.[188] C. Xu, Y. Su, D. Liu, X. He, Phys. Chem. Chem. Phys. 2015, 17,

25440.[189] I. M. Patil, M. Lokanathan, B. Kakade, J. Mater. Chem. A 2016, 4,

4506.[190] H. Jiang, Y. Zhu, Q. Feng, Y. Su, X. Yang, C. Li, Chem. – Eur.

J. 2014, 20, 3106.[191] J. Wang, Z. X. Wu, L. L. Han, Y. Y. Liu, J. P. Guo, H. L. Xin,

D. L. Wang, Chin. Chem. Lett. 2016, 27, 597.[192] H. Jiang, Y. Wang, J. Hao, Y. Liu, W. Li, J. Li, Carbon 2017, 122, 64.[193] Y. Qiu, L. Xin, F. Jia, J. Xie, W. Li, Langmuir 2016, 32, 12569.[194] S. Fu, C. Zhu, J. Song, M. H. Engelhard, B. Xiao, D. Du, Y. Lin,

Chem. – Eur. J. 2017, 23, 10460.[195] Y. Jiang, L. Yang, T. Sun, J. Zhao, Z. Lyu, O. Zhuo, X. Wang, Q. Wu,

J. Ma, Z. Hu, ACS Catal. 2015, 5, 6707.[196] H. Zhao, C. Sun, Z. Jin, D. W. Wang, X. Yan, Z. Chen, G. Zhu,

X. Yao, J. Mater. Chem. A 2015, 3, 11736.[197] D. Yan, Y. Li, J. Huo, R. Chen, L. Dai, S. Wang, Adv. Mater. 2017,

29, 1606459.[198] C. Tang, H. F. Wang, Q. Zhang, Acc. Chem. Res. 2018, 51, 881.[199] B. Q. Li, C. Tang, H. F. Wang, X. L. Zhu, Q. Zhang, Sci. Adv. 2016,

2, e1600495.[200] H. Jin, H. Huang, Y. He, X. Feng, S. Wang, L. Dai, J. Wang, J. Am.

Chem. Soc. 2015, 137, 7588.[201] E. Antolini, Renewable Sustainable Energy Rev. 2016, 58, 34.[202] R. Liu, H. Zhang, S. Liu, X. Zhang, T. Wu, X. Ge, Y. Zang, H. Zhao,

G. Wang, Phys. Chem. Chem. Phys. 2016, 18, 4095.[203] B. Wang, S. Li, X. Wu, J. Liu, J. Chen, J. Mater. Chem. A 2016, 4,

11789.[204] J. Zhang, H. Zhou, X. Liu, J. Zhang, T. Peng, J. Yang, Y. Huang,

S. Mu, J. Mater. Chem. A 2016, 4, 15870.[205] H. Ba, Y. Liu, L. T. Phuoc, C. D. Viet, J. M. Nhut, D. L. Nguyen, O. Ersen,

G. Tuci, G. Giambastiani, C. P. Huu, ACS Catal. 2016, 6, 1408.[206] Y. Yang, T. Liu, Q. Liao, D. Ye, X. Zhu, J. Li, P. Zhang, Y. Peng,

S. Chen, Y. Li, J. Mater. Chem. A 2016, 4, 15913.[207] M. Gong, H. Dai, Nano Res. 2015, 8, 23.[208] M. Busch, N. B. Halck, U. I. Kramm, S. Siahrostami, P. Krtil,

J. Rossmeisl, Nano Energy 2016, 29, 126.[209] M. Tahir, L. Pan, F. Idrees, X. Zhang, L. Wang, J. J. Zou, Z. L. Wang,

Nano Energy 2017, 37, 136.[210] M. Gong, Y. Li, H. Wang, Y. Liang, J. Z. Wu, J. Zhou, J. Wang,

T. Regier, F. Wei, H. Dai, J. Am. Chem. Soc. 2013, 135, 8452.[211] Z. Xia, Nat. Energy 2016, 1, 16155.[212] Y. Zhao, R. Nakamura, K. Kamiya, S. Nakanishi1, K. Hashimoto,

Nat. Commun. 2013, 4, 2390.[213] T. Y. Ma, S. Dai, M. Jaroniec, S. Z. Qiao, Angew. Chem., Int. Ed.

2014, 53, 7281.[214] S. Chen, J. Duan, M. Jaroniec, S. Z. Qiao, Adv. Mater. 2014, 26,

2925.[215] X. Yu, M. Zhang, J. Chen, Y. Li, G. Shi, Adv. Energy Mater. 2016, 6,

1501492.[216] J. Zhao, Y. Liu, X. Quan, S. Chen, H. Zhao, H. Yu, Electrochim. Acta

2016, 204, 169.



Adv. Mater. 2019, 1805598

[217] Y. P. Zhu, Y. Jing, A. Vasileff, T. Heine, S. Z. Qiao, Adv. Energy Mater. 2017, 7, 1602928.

[218] T. Y. Ma, J. Ran, S. Dai, M. Jaroniec, S. Z. Qiao, Angew. Chem., Int. Ed. 2015, 54, 4646.

[219] K. J. Lee, Y. J. Sa, H. Y. Jeong, C. W. Bielawski, S. H. Joo, H. R. Moon, Chem. Commun. 2015, 51, 6773.

[220] Z. Liu, Z. Zhao, Y. Wang, S. Dou, D. Yan, D. Liu, Z. Xia, S. Wang, Adv. Mater. 2017, 29, 1606207.

[221] H. W. Park, D. U. Lee, Y. Liu, J. Wu, L. F. Nazar, Z. Chen, J. Electro-chem. Soc. 2013, 160, A2244.

[222] X. Li, Y. Fang, S. Zhao, J. Wu, F. Li, M. Tian, X. Long, J. Jin, J. Ma, J. Mater. Chem. A 2016, 4, 13133.

[223] L. N. Han, X. Wei, B. Zhang, X. H. Li, Q. C. Zhu, K. X. Wang, J. S. Chen, RSC Adv. 2016, 6, 56765.

[224] S. Patra, R. Choudhary, E. Roy, R. Madhuri, P. K. Sharma, Nano Energy 2016, 30, 118.

[225] X. Huang, Q. Wang, D. Jiang, Y. Huang, Catal. Commun. 2017, 100, 89.

[226] Z. Pei, H. Li, Y. Huang, Q. Xue, Y. Huang, M. Zhu, Z. Wang, C. Zhi, Energy Environ. Sci. 2017, 10, 742.

[227] L. Garcia, in Compendium of Hydrogen Energy (Eds: V. Subramani, A. Basile, T. N. Veziro), Woodhead Publishing Series in Energy, Woodhead Publishing, Sawston, Cambridge, UK 2015, pp. 83–107.

[228] R. Paul, R. G. Reifenberger, T. S. Fisher, D. Y. Zemlyanov, Chem. Mater. 2015, 27, 5915.

[229] W. Zhou, J. Jia, J. Lu, L. Yang, D. Hou, G. Li, S. Chen, Nano Energy 2016, 28, 29.

[230] B. Conway, B. Tilak, Electrochim. Acta 2002, 47, 3571.[231] N. Mahmood, Y. Yao, J. W. Zhang, L. Pan, X. Zhang, J. J. Zou, Adv.

Sci. 2018, 5, 1700464.[232] P. Sudhagar, N. Roy, R. Vedarajan, A. Devadoss, C. Terashima,

K. Nakata, A. Fujishima, in Photoelectrochemical Solar Fuel Production: From Basic Principles to Advanced Devices (Eds: S. Giménez, J. Bisquert) Springer International Publishing, Cham, Switzerland 2016, pp. 105–160.

[233] Y. S. Jun, J. Park, S. U. Lee, A. Thomas, W. H. Hong, G. D. Stucky, Angew. Chem., Int. Ed. 2013, 52, 11083.

[234] S. N. Talapaneni, J. Kim, S. H. Je, O. Buyukcakir, J. Oh, A. Coskun, J. Mater. Chem. A 2017, 5, 12080.

[235] K. Xie, H. Wu, Y. Meng, K. Lu, Z. Wei, Z. Zhang, J. Mater. Chem. A 2015, 3, 78.

[236] Y. Tiana, Y. Ye, X. Wang, S. Peng, Z. Wei, X. Zhang, W. Liu, Appl. Catal., A 2017, 529, 127.

[237] Y. Zhao, F. Zhao, X. Wang, C. Xu, Z. Zhang, G. Shi, L. Qu, Angew. Chem. 2014, 126, 14154.

[238] J. Duan, S. Chen, M. Jaroniec, S. Z. Qiao, ACS Nano 2015, 9, 931.[239] D. Yan, S. Dou, L. Tao, Z. Liu, Z. Liu, J. Huo, S. Wang, J. Mater.

Chem. A 2016, 4, 13726.[240] S. S. Shinde, A. Sami, J. H. Lee, ChemCatChem 2015, 7, 3873.[241] J. Zhang, Z. Zhao, Z. Xia, L. Dai, Nat. Nanotechnol. 2015, 10, 444.[242] Q. Liu, Y. Wang, L. Dai, J. Yao, Adv. Mater. 2016, 28, 3000.[243] X. Yan, Y. Jia, T. Odedairo, X. Zhao, Z. Jin, Z. Zhu, X. Yao, Chem.

Commun. 2016, 52, 8156.[244] Z. Peng, S. Yang, D. Jia, P. Da, P. He, A. M. Al-Enizi, G. Ding,

X. Xie, G. Zheng, J. Mater. Chem. A 2016, 4, 12878.[245] J. Zhang, L. Dai, ACS Catal. 2015, 5, 7244.[246] J. Lai, S. Li, F. Wu, M. Saqib, R. Luque, G. Xu, Energy Environ. Sci.

2016, 9, 1210.

[247] B. Kumar, M. Asadi, D. Pisasale, S. S. Ray, B. A. Rosen, R. Haasch, J. Abiade, A. L. Yarin, A. S. Khojin, Nat. Commun. 2013, 4, 2819.

[248] P. P. Sharma, J. Wu, R. M. Yadav, M. Liu, C. J. Wright, C. S. Tiwary, B. I. Yakobson, J. Lou, P. M. Ajayan, X. D. Zhou, Angew. Chem., Int. Ed. 2015, 54, 13701.

[249] N. Sreekanth, M. A. Nazrulla, T. V. Vineesh, K. Sailaja, K. L. Phani, Chem. Commun. 2015, 51, 16061.

[250] S. Zhang, P. Kang, S. Ubnoske, M. K. Brennaman, N. Song, R. L. House, J. T. Glass, T. J. Meyer, J. Am. Chem. Soc. 2014, 136, 7845.

[251] Y. Liu, S. Chen, X. Quan, H. Yu, J. Am. Chem. Soc. 2015, 137, 11631.

[252] W. Li, M. Seredych, E. R. Castellun, T. J. Bandosz, ChemSusChem 2016, 9, 606.

[253] S. J. Amirfakhri, D. Binny, J. L. Meunier, D. Berk, J. Power Sources 2014, 257, 356.

[254] P. Wu, P. Du, H. Zhang, C. Cai, Phys. Chem. Chem. Phys. 2013, 15, 6920.

[255] T. Zhang, C. Li, Y. Gu, X. Yan, B. Zheng, Y. Li, H. Liu, N. Lu, Z. Zhang, G. Feng, Talanta 2017, 165, 143.

[256] V. Broje, A. A. Keller, Environ. Sci. Technol. 2006, 40, 7914.[257] M. A. Zahed, H. A. Aziz, M. H. Isa, L. Mohajeri, S. Mohajeri,

Bioresour. Technol. 2010, 101, 9455.[258] S. Wan, H. C. Bi, L. T. Sun, Nanotechnol. Rev. 2016, 5, 3.[259] Z. Q. Lin, Z. P. Zeng, X. C. Gui, Z. K. Tang, M. Zou, A. Cao,

Adv. Energy Mater. 2016, 6, 1600554.[260] H. Liu, H. Wang, X. Zhang, Adv. Mater. 2015, 27, 249.[261] X. Wang, X. Lu, B. Liu, D. Chen, Y. Tong, G. She, Adv. Mater. 2014,

26, 4763.[262] X. Wang, B. Liu, R. Liu, Q. Wang, X. Hou, D. Chen, R. Wang,

G. Shen, Angew. Chem., Int. Ed. 2014, 53, 1849.[263] W. Li, X. Xu, C. Liu, M. C. Tekell, J. Ning, J. Guo, J. Zhang, D. Fan,

Adv. Funct. Mater. 2017, 27, 1702738.[264] J. H. Montoya, L. C. Seitz, P. Chakthranont, A. Vojvodic,

T. F. Jaramillo, J. K. Nørskov, Nat. Mater. 2017, 16, 70.[265] M. R. Lukatskaya, B. Dunn, Y. Gogotsi, Nat. Commun. 2016, 7,

12647.[266] F. Podjaski, J. Kröger, B. V. Lotsch, Adv. Mater. 2018, 30, 1705477.[267] D. Yu, K. Goh, H. Wang, L. Wei, W. Jiang, Q. Zhang, L. Dai,

Y. Chen, Nat. Nanotechnol. 2014, 9, 555.[268] X. Wang, K. Jiang, G. Shen, Mater. Today 2015, 18, 265.[269] C. Hu, X. Chen, Q. Dai, M. Wang, L. Qu, L. Dai, Nano Energy 2017,

41, 367.[270] H. Sun, X. You, J. Deng, X. Chen, Z. Yang, P. Chen, X. Fang,

H. Peng, Angew. Chem., Int. Ed. 2014, 53, 1.[271] Z. Zhang, X. Chen, P. Chen, G. Guan, L. Qiu, H. Lin, Z. Yang,

W. Bai, Y. Luo, H. Peng, Adv. Mater. 2014, 26, 466.[272] R. Paul, A. Roy, L. Dai, in Carbon-Based Metal-Free Catalysts: Design

and Applications (Ed: L. Dai), Wiley, New York 2018, Ch. 7.[273] L. Yang, D. Cheng, H. Xu, X. Zeng, X. Wan, J. Shui, Z. Xiang,

D. Cao, Proc. Natl. Acad. Sci. USA 2018, 115, 6626.[274] H. B. Yang, S. F. Hung, S. Liu, K. Yuan, S. Miao, L. Zhang,

X. Huang, H. Y. Wang, W. Cai, R. Chen, J. Gao, X. Yang, W. Chen, Y. Huang, H. M. Chen, C. M. Li, T. Zhang, B. Liu, Nat. Energy 2018, 3, 140.

[275] R. Ye, J. Dong, L. Wang, R. M. Cruz, Y. Li, P. F. An, M. J. Yacamán, B. I. Yakobson, D. Chen, J. M. Tour, Carbon 2018, 132, 623.

[276] Q. Liu, X. Liu, L. Zheng, J. Shui, Angew. Chem. 2018, 130, 1218.

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